x/tools/gopls: filter out overlays in the module cache

[findleyr]

In #71075, it is suggested that the Go command return errors if overlays are in the module cache.
Right now, gopls may construct such overlays if the user opens a module cache file (which they would do when jumping to definition).

We should filter out overlays for module cache files. If the user edits a module cache file, we should surface an error.

Putting this in the v0.19.0 milestone, to be done before the release of Go 1.25.

Gopls can definitely filter out overlays from the module cache (perhaps reporting an error back to the user). Then it would be safe for the Go command to fail in that case.

Originally posted by @findleyr in #71075

[gabyhelp]

Related Issues

• cmd/go: support overlays in the go command #39958 (closed)

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