x/tools/gopls: incorrect metadata invalidation when changing package name

[findleyr]

Gopls doesn't correctly invalidate package data when moving a file to a new package:
https://cs.opensource.google/go/x/tools/+/master:internal/lsp/cache/snapshot.go;l=1842;drc=37590b385d564de6d3b0aedae2b5821b3f357d1d

As a result, it's pretty easy to break gopls' metadata by toggling package names. Repro: move a file from package x_test to package x, wait a second, then move it back to package x_test: the resulting file will not be contained in any package...

Filing this issue to track the fix, and to associate with the new gopls/metadata label.

[gopherbot]

Change https://go.dev/cl/401795 mentions this issue: internal/lsp/cache: invalidate packages that have added files