cmd/go/internal/modload: treat \ as a path separator instead of a shell special character

According to the code comment, \ should be treated as a path separator.
See fileNameOK in golang.org/x/mod/module/module.go.

Change-Id: I05e05246005191ecccfecf46848aba5cb1359956
Reviewed-on: https://go-review.googlesource.com/c/go/+/310429


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Robert Findley <rfindley@google.com>