cmd/go/internal/modload: support go.mod retract directive
The go command now recognizes 'retract' directives in go.mod. A
retract directive may be used by a module author to indicate a
version should not be used. The go command will not automatically
upgrade to a retracted version. Retracted versions will not be
considered when resolving version queries like "latest" that don't
refer to a specific version.
Internally, when the go command resolves a version query, it will find
the highest release version (or pre-release if no release is
available), then it will load retractions from the go.mod file for
that version. Comments on retractions are treated as a rationale and
may appear in error messages. Retractions are only loaded when a query
is resolved, so this should have no impact on performance for most
builds, except when go.mod is incomplete.
For #24031
Change-Id: I17d643b9e03a3445676dbf1a5a351090c6ff6914
Reviewed-on: https://go-review.googlesource.com/c/go/+/228380
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com>