x/tools/gopls: support gomvpkg as a command

[stamblerre]

See https://pkg.go.dev/golang.org/x/tools/cmd/gomvpkg?tab=doc.

[gopherbot]

Change https://go.dev/cl/425955 mentions this issue: gopls: Add support for gomvpkg as a command.

[findleyr]

I think once #41567 is complete, there will be less need for implementing package renaming as a command, though gomvpkg still supports features that simple package renaming will not (at least in its initial revision).

We should reconsider how this interacts with package renaming via the textDocument/rename request: what are the critical features provided by gomvpkg, and how can we best expose them through the LSP.

For example, one feature that the current package renaming implementation will not support is moving a package to an arbitrary other location: we can move foo/a to foo/b by renaming a->b in the package declaration package a, but we can't move foo/a to internal/a. Should we support this via a custom command (as is suggested by this issue), or by "hacks" on top of textDocument/rename, for example renaming a->../internal/a.