Filter out metapackages from assets builder

[kiironoaki]

Package with type metapackage doesn't have installation path, which brings with it a problem finding discovery JSON files in all packages. Filter add for removing "metapackages" and leave only real

Refs: https://github.com/thecodingmachine/discovery/issues/17

Refs: https://getcomposer.org/doc/04-schema.md#type