Over time there may exist two modules with names that differ only in case.
On systems with case-insensitive file systems, we need to make sure those
modules do not collide in the download cache.

Do this by using the new "safe encoding" for file system paths as well as
proxy paths.

Fixes #25992.

Change-Id: I717a9987a87ad5c6927d063bf30d10d9229498c9
Reviewed-on: https://go-review.googlesource.com/124379
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>