x/tools/gopls: go list diagnostics do not work for individual modules when go.work is used

[findleyr]

gopls has various logic that extracts information from the original go list invocation (see goCommandDiagnostics). In particular, it looks for errors related to go.sum entries.

When using a go.work file, (1) the go.work.sum is automatically updated, contrary to the behavior with respect to go.sum files, and (2) gopls does not get errors about individual go.sum files that may be out of sync.

Low priority, but we should resolve this inconsistency. Ideally, we could separate the calculation of these diagnostics from the actual go/packages.Load invocation. Having them coupled is problematic.

CC @adonovan @bcmills