…the relevant module path, not abstract semver

In CL 279394 we started validating that versions in "exclude" and
"retract" directives are canonical. Unfortunately, we use the semver
package's notion of canonicalness, and the semver package doesn't know
anything about +incompatible versions or major-version suffixes.

The resulting error messages also don't indicate an appropriate fix if
the problem is that the user forgot either the "+incompatible" suffix
on the version string or the "/vN" suffix on the module path.

This change corrects both of those problems by validating the version
against the corresponding module path. (For "exclude" directives, that
is the module path to be excluded; for "retract" directives, it is the
module declared in the "module" directive of the same go.mod file.)

Updates golang/go#44497
For golang/go#44498

Change-Id: I39732d79c3ab3a43bb1fb8905062fe6cb26d3edc
Reviewed-on: https://go-review.googlesource.com/c/mod/+/295089
Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
(cherry picked from commit 66f6d92)
Reviewed-on: https://go-review.googlesource.com/c/mod/+/295931
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

…t -exclude'

The fix is to pull in CL 295931 from the x/mod repo.

Updates #44497
Fixes #44498

Change-Id: I008b58d0f4bb48c09d4f1e6ed31d11a714f87dc0
Reviewed-on: https://go-review.googlesource.com/c/go/+/295150
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
(cherry picked from commit 691ac80)
Reviewed-on: https://go-review.googlesource.com/c/go/+/295930