cmd/go/internal/modcmd: loosen path validation in "go mod edit"

cmd/go/internal/modcmd: loosen path validation in "go mod edit"

Replaced modules require only valid import paths, not full
module paths that can be fetched with 'go get'.

The 'go' command does not in general reject manually-edited go.mod
files with these paths, so 'go mod edit' should not reject them
either.

Fixes #30513

Change-Id: I4f1a5c65937f91d41478f8d218c8018e0c70f320
Reviewed-on: https://go-review.googlesource.com/c/go/+/210343
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>