compiler: mangle dots in pkgpath

compiler: mangle dots in pkgpath

    We need to mangle dots to avoid problems with -fgo-pkgpath=a.0.
    That will confuse the name mangling, which assumes that names
    entering the mangling cannot contain arbitrary dot characters.
    We don't need to mangle other characters; go_encode_id will handle them.

    Fixes golang/go#33871

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/200838

From-SVN: r276913