module: Add module specific symbol namespace support

Designate the "module:${modname}" symbol namespace to mean: 'only
export to the named module'.

Notably, explicit imports of anything in the "module:" space is
forbidden.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>