x/tools/gopls: deprecate the "experimentalUseInvalidMetadata" setting

[findleyr]

Discovered while investigating #54147: with "experimentalUseInvalidMetadata" set, we may diagnose a package with invalid metadata, and not re-diagnose when new metadata arrives. This can lead to diagnostics which are stale until you begin typing again.

We could fix this by re-diagnosing after each metadata load, but more generally we should reconsider whether it is worth landing this feature:

Pros:

• when the workspace is totally broken, gopls will continue to work

Cons:

• though gopls will continue to work, it may have inconsistent or confusing state
• users should generally fix broken workspaces before doing anything else
• we really don't want users to have to restart gopls, so in all other respects we are trying to avoid places where gopls "remembers history"

[gopherbot]

Change https://go.dev/cl/420956 mentions this issue: internal/lsp/cache: invalid packages should not be workspace packages

[findleyr]

CC @adonovan

Unless anyone objects, I propose that we remove this setting. We should avoid any cases where gopls' state is path-dependent.

[gopherbot]

Change https://go.dev/cl/434640 mentions this issue: gopls: deprecate three experimental features

[gopherbot]

Change https://go.dev/cl/452057 mentions this issue: gopls/internal/lsp/cache: eliminate obsolete invalidation step