cmd/go/internal/modfetch: avoid path.Join in URL errors
path.Join collapses duplicated '/' tokens, but an HTTP URL
intentionally includes a '://' after the scheme.
This should fix the syntax of the errors seen in
https://build.golang.org/log/
a17d0c7b6159ea4dad0a9e5378ab5a36ee30ce44.
Updates #52727.
Change-Id: I6e1773a7eb8b3e7e2b3ca29540c1e94a7cd2d99d
Reviewed-on: https://go-review.googlesource.com/c/go/+/461682
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>