[release-branch.go1.16] cmd/go: avoid +incompatible major versions if a go.mod...

[release-branch.go1.16] cmd/go: avoid +incompatible major versions if a go.mod file exists in a subdirectory for that version

Previous versions of the 'go' command would reject a pseudo-version
passed to 'go get' if that pseudo-version had a mismatched major
version and lacked a "+incompatible" suffix. However, they would
erroneously accept a version *with* a "+incompatible" suffix even if
the repo contained a vN/go.mod file for the same major version, and
would generate a "+incompatible" pseudo-version or version if the user
requested a tag, branch, or commit hash.

This change uniformly rejects "vN.…" without "+incompatible", and also
avoids resolving to "vN.…+incompatible", when vN/go.mod exists.
To maintain compatibility with existing go.mod files, it still accepts
"vN.…+incompatible" if the version is requested explicitly as such
and the repo root lacks a go.mod file.

Fixes #51331
Updates #51324
Updates #36438

Change-Id: I2b16150c73fc2abe4d0a1cd34cb1600635db7139
Reviewed-on: https://go-review.googlesource.com/c/go/+/387675


Trust: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
(cherry picked from commit 5a9fc946)
Reviewed-on: https://go-review.googlesource.com/c/go/+/387923


Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>