cmd/go: open correct path when loading embeds from root directory

cmd/go: open correct path when loading embeds from root directory

The existing implementation of `load.resolveEmbed`
uses an expression like `path[len(pkgdir)+1:]`.
Though the `+1` is intended to remove a prefix slash,
the expression returns an incorrect path when `pkgdir`
is "/". (ex.: when removing "/" from "/foo", want "foo",
but got "oo")

It seems that `str.TrimFilePathPrefix` would solve
the problem, but the function contains the same bug.

So, this commit fixes `str.TrimFilePathPrefix` then
applies it to `load.resolveEmbed` to solve the issue.
The fix is quite simple. First, remove prefix. Then
check whether the remained first letter is equal to
`filepath.Separator`. If so, remove it then return.

Fixed #49570

Change-Id: I26ab727ee4dfcbf51ed9bd0a573957ced2154515
Reviewed-on: https://go-review.googlesource.com/c/go/+/396694
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>