Support for conformance-type of the ietf-yang-library

In the YANG module library https://tools.ietf.org/html/rfc7895 there is a field called 'conformance-type' with 2 possible values.   The values basically mean:

• "implement": the server supports the full module
• "import": the server does not support the actual top level data nodes of the module, only the imports, groupings, ...

It would be useful if we could pass the list of "only-import" modules to the YANG parser, and that the YANG parser would then automatically ignore the top level data nodes of those modules.