* feat(versioning): add a `gomod` versioning
First step of #44184: a versioning for Go modules that behaves like `semver`
for now and is not the default anywhere yet, so it can be tried on repositories
before the `go` datasource switches to it.
It exports `isPseudoVersion()`, which follows `IsPseudoVersion()` from
golang.org/x/mod/module and recognises all three pseudo-version forms, as the
place for Go version helpers that do not belong to the `go` directive's
`go-mod-directive` versioning.
* refactor(versioning/gomod): accept an undefined version in isPseudoVersion()
Review feedback on #46145: the callers check optional values such as
`upgrade.newValue`, so `isPseudoVersion()` takes `undefined` and answers
`false` for it, and they no longer need their own guard.
* refactor(versioning/gomod): return early for an undefined version in isPseudoVersion()
Sebastian Poxhofer