x/tools/gopls: adding a missing package does not invalidate missing package errors

[findleyr]

Following up on #53878, we don't have an issue tracking the bug represented by gopls/internal/regtest/watch.TestCreateDependency: adding a missing package does not invalidate missing package errors.

[gopherbot]

Change https://go.dev/cl/419501 mentions this issue: internal/lsp/cache: invalidate packages with missing deps when files are