For a package in the module root, using the containing directory name
might mean the directory in the module cache, in which case the
executable has a final @v1.2.3 in it, which is no good. Fix that.

While we're here, change go install example.com/cmd/foo/v2 to
install foo instead of the less useful "v2".

Fixes #24667.
Fixes #26869.

Change-Id: Ie40ca1bc9e27955441f1cdb7abd3a1f69034c9f5
Reviewed-on: https://go-review.googlesource.com/128900Reviewed-by: Bryan C. Mills <bcmills@google.com>