On Windows, GOMODCACHE almost never starts with a slash, and
"go doc -http" constructs a GOPROXY URL by doing "file://" + GOMODCACHE,
resulting in an invalid file URI.
For example, if GOMODCACHE is "C:\foo", then the file URI should be
"file:///C:/foo", but it becomes "file://C:/foo" instead, where "C:" is
understood as a host name, not a drive letter.
Fixes #74137.
Change-Id: I23e776e0f649a0062e01d1a4a6ea8268ba467331
Reviewed-on: https://go-review.googlesource.com/c/go/+/684575
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Michael Matloob <matloob@google.com>