proposal: cmd/go: 'go mod tidy' should remove stale 'exclude' directives

[bcmills]

exclude directives in the go.mod file prevent commands such as go get -u from selecting particular versions of modules.

However, if the active version of a dependency is already semantically higher than the excluded version, go get -u would never select that version implicitly in the first place (depending on the resolution of #26474). The only time it could be introduced is by an explicit downgrade of some package, and explicit downgrades — though possible — are likely to be rare in practice.

That makes those exclude directives noisy and somewhat misleading: they appear to have an effect on version selection, but may actually be very stale and completely irrelevant.

I propose that go mod tidy should remove exclude directives if the selected version of the module in question is already higher than the excluded version.

(CC @rsc @jayconrod)