x/tools/gopls: improve experience when editing a go.mod file by hand

[stamblerre]

Right now, we invalidate metadata on every keystroke, and for a go.mod file, that means that the metadata for the entire workspace is invalidated as you type. This triggers a go list on every keystroke, causing CPU usage to spike. We need to improve the UX of editing a go.mod file by hand, and that likely will be resolved by only invalidating metadata on go.mod saves, not edits.

#42266 might also be helpful, though I don't think it's the critical issue.

[gopherbot]

Change https://golang.org/cl/271299 mentions this issue: internal/lsp/cache: only invalidate metadata for go.mod files on save

[gopherbot]

Change https://golang.org/cl/279033 mentions this issue: gopls/internal/regtest: test metadata validation only on save for go.mod

[gopherbot]

Change https://golang.org/cl/279720 mentions this issue: internal/lsp/cache: only reload the workspace on saved changes