update package deps for custom project root

[vanvoorden]

Two small improvements to package manifest:

• We follow the pattern from swift-corelibs-foundation¹ to compute the location of dependencies:
  • SWIFTCI_USE_LOCAL_DEPS=1 moves us up one directory when looking for dependencies.
  • Missing or empty SWIFTCI_USE_LOCAL_DEPS pulls dependencies from GitHub.
• Our swift-collections dependency from GitHub pulls from main. This is consistent with what we choose for swift-foundation-icu² and swift-syntax³.

Tested locally from my machine:

$ cd ~/Developer/swift-foundation
$ swift build
...
Build complete!

$ SWIFTCI_USE_LOCAL_DEPS=~/Developer swift build
...
Build complete!

$ SWIFTCI_USE_LOCAL_DEPS=~ swift build
...
error: the package at '/Users/rick/swift-collections' cannot be accessed (Error Domain=NSCocoaErrorDomain Code=260 "The folder “swift-collections” doesn’t exist." UserInfo={NSUserStringVariant=(
    Folder
), NSFilePath=/Users/rick/swift-collections, NSURL=file:///Users/rick/swift-collections, NSUnderlyingError=0x6000020b0e40 {Error Domain=NSOSStatusErrorDomain Code=-43 "fnfErr: File not found"}})
error: ExitCode(rawValue: 1)

Footnotes

1. https://github.com/swiftlang/swift-corelibs-foundation/blob/swift-6.1.2-RELEASE/Package.swift#L155-L181 ↩

2. https://github.com/swiftlang/swift-foundation/pull/802 ↩

3. https://github.com/swiftlang/swift-foundation/pull/1025 ↩