`coverage combine` cannot find local plugin files

[Jacobfaib]

Describe the bug
Consider a project:

pyproject.toml
my_local_plugin.py
src/
  foo.py

# pyproject.toml
[tool.coverage.run]
plugins = ["my_local_plugin"]

# my_local_plugin.py
def coverage_init(*args, **kwargs):
    pass

And then

# From project root directory (location of pyproject.toml)
$ coverage run foo.py
$ coverage combine
Traceback (most recent call last):
  File "/path/to/venv/bin/coverage", line 8, in <module>
    sys.exit(main())
             ~~~~^^
  File "/path/to/venv/lib/python3.13/site-packages/coverage/cmdline.py", line 970, in main
    status = CoverageScript().command_line(argv)
  File "/path/to/venv/lib/python3.13/site-packages/coverage/cmdline.py", line 681, in command_line
    return self.do_run(options, args)
           ~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/path/to/venv/lib/python3.13/site-packages/coverage/cmdline.py", line 855, in do_run
    self.coverage.start()
    ~~~~~~~~~~~~~~~~~~~^^
  File "/path/to/venv/lib/python3.13/site-packages/coverage/control.py", line 660, in start
    self._init()
    ~~~~~~~~~~^^
  File "/path/to/venv/lib/python3.13/site-packages/coverage/control.py", line 365, in _init
    self._plugins.load_from_config(self.config.plugins, self.config)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/venv/lib/python3.13/site-packages/coverage/plugin_support.py", line 48, in load_from_config
    __import__(module)
    ~~~~~~~~~~^^^^^^^^
ModuleNotFoundError: No module named 'my_local_plugin'

To Reproduce
How can we reproduce the problem? Please be specific. Don't link to a failing CI job. Answer the questions below:

1. What version of Python are you using? 3.13
2. What version of coverage.py shows the problem? The output of coverage debug sys is helpful.

$ coverage debug sys
-- sys -------------------------------------------------------
               coverage_version: 7.8.2
                coverage_module: /Users/jfaibussowit/soft/nv/legate.core.internal/venv/lib/python3.13/site-packages/coverage/__init__.py
                           core: -none-
                        CTracer: available from /Users/jfaibussowit/soft/nv/legate.core.internal/venv/lib/python3.13/site-packages/coverage/tracer.cpython-313-darwin.so
           plugins.file_tracers: -none-
            plugins.configurers: -none-
      plugins.context_switchers: -none-
              configs_attempted: /Users/jfaibussowit/soft/nv/.coveragerc
                                 /Users/jfaibussowit/soft/nv/setup.cfg
                                 /Users/jfaibussowit/soft/nv/tox.ini
                                 /Users/jfaibussowit/soft/nv/pyproject.toml
                   configs_read: -none-
                    config_file: None
                config_contents: -none-
                      data_file: -none-
                         python: 3.13.3 (main, Apr  8 2025, 13:54:08) [Clang 17.0.0 (clang-1700.0.13.3)]
                       platform: macOS-15.4.1-arm64-arm-64bit-Mach-O
                 implementation: CPython
                    gil_enabled: True
                     executable: /Users/jfaibussowit/soft/nv/legate.core.internal/venv/bin/python
                   def_encoding: utf-8
                    fs_encoding: utf-8
                            pid: 63286
                            cwd: /Users/jfaibussowit/soft/nv
                           path: /Users/jfaibussowit/soft/nv/legate.core.internal/venv/bin
                                 /opt/homebrew/Cellar/python@3.13/3.13.3_1/Frameworks/Python.framework/Versions/3.13/lib/python313.zip
                                 /opt/homebrew/Cellar/python@3.13/3.13.3_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13
                                 /opt/homebrew/Cellar/python@3.13/3.13.3_1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/lib-dynload
                                 /Users/jfaibussowit/soft/nv/legate.core.internal/venv/lib/python3.13/site-packages
                    environment: HOME = /Users/jfaibussowit
                                 PYTHONBREAKPOINT = ipdb.set_trace
                   command_line: /Users/jfaibussowit/soft/nv/legate.core.internal/venv/bin/coverage debug sys
         sqlite3_sqlite_version: 3.49.2
             sqlite3_temp_store: 0
        sqlite3_compile_options: ATOMIC_INTRINSICS=1, COMPILER=clang-17.0.0, DEFAULT_AUTOVACUUM,
                                 DEFAULT_CACHE_SIZE=-2000, DEFAULT_FILE_FORMAT=4,
                                 DEFAULT_JOURNAL_SIZE_LIMIT=-1, DEFAULT_MMAP_SIZE=0, DEFAULT_PAGE_SIZE=4096,
                                 DEFAULT_PCACHE_INITSZ=20, DEFAULT_RECURSIVE_TRIGGERS,
                                 DEFAULT_SECTOR_SIZE=4096, DEFAULT_SYNCHRONOUS=2,
                                 DEFAULT_WAL_AUTOCHECKPOINT=1000, DEFAULT_WAL_SYNCHRONOUS=2,
                                 DEFAULT_WORKER_THREADS=0, DIRECT_OVERFLOW_READ, ENABLE_API_ARMOR,
                                 ENABLE_COLUMN_METADATA, ENABLE_DBSTAT_VTAB, ENABLE_FTS3,
                                 ENABLE_FTS3_PARENTHESIS, ENABLE_FTS5, ENABLE_MATH_FUNCTIONS,
                                 ENABLE_MEMORY_MANAGEMENT, ENABLE_PREUPDATE_HOOK, ENABLE_RTREE,
                                 ENABLE_SESSION, ENABLE_STAT4, ENABLE_UNLOCK_NOTIFY, MALLOC_SOFT_LIMIT=1024,
                                 MAX_ATTACHED=10, MAX_COLUMN=2000, MAX_COMPOUND_SELECT=500,
                                 MAX_DEFAULT_PAGE_SIZE=8192, MAX_EXPR_DEPTH=1000, MAX_FUNCTION_ARG=1000,
                                 MAX_LENGTH=1000000000, MAX_LIKE_PATTERN_LENGTH=50000,
                                 MAX_MMAP_SIZE=0x7fff0000, MAX_PAGE_COUNT=0xfffffffe, MAX_PAGE_SIZE=65536,
                                 MAX_SQL_LENGTH=1000000000, MAX_TRIGGER_DEPTH=1000,
                                 MAX_VARIABLE_NUMBER=250000, MAX_VDBE_OP=250000000, MAX_WORKER_THREADS=8,
                                 MUTEX_PTHREADS, SYSTEM_MALLOC, TEMP_STORE=1, THREADSAFE=1, USE_URI

Expected behavior
coverage should be able to import the package, using the current working directory. Setting PYTHONPATH=$(pwd) then of course it works.