scripts/nsdeps: support nsdeps for external module builds

scripts/nsdeps: support nsdeps for external module builds

scripts/nsdeps is written to take care of only in-tree modules.
Perhaps, this is not a bug, but just a design. At least,
Documentation/core-api/symbol-namespaces.rst focuses on in-tree modules.

Having said that, some people already tried nsdeps for external modules.
So, it would be nice to support it.

Reported-by: Steve French <[email protected]>
Reported-by: Jessica Yu <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>
Tested-by: Jessica Yu <[email protected]>
Acked-by: Jessica Yu <[email protected]>
Reviewed-by: Matthias Maennich <[email protected]>
Tested-by: Matthias Maennich <[email protected]>