cmd/go: use Join functions instead of adding path separators to strings

cmd/go: use Join functions instead of adding path separators to strings

Adding a file path separator is incorrect for a file path that may be
the root directory on a Unix platform (such as in a container or
chroot).

Adding a path separator is incorrect for a package path prefix that
may be the empty string (as in the "std" module in GOROOT/src).

And in both cases, a Join function is arguably clearer and simpler
anyway.

Fixes #51506 (maybe).

Change-Id: Id816930811ad5e4d1fbd206cddf219ecd4ad39a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/463178
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>