Join yang resource providers from restconf-nb and yanglib into single service

Currently there two web services with own endpoints providing yang sources by module/submodule name and revision:

RestconfSchemaService(Impl) of restconf-nb

• path:  (host:port)/rests/modules/(name)/(revision)
• content-types: application/yang, application/xml+yin
• source: DOMSchemaService instance

YangLibService(YangLibProvider) of yanglib

• path: (host:port)/yanglib/schemas/(name)/(revision)
• content-types: application/yang, text/plain
• source: local [karaf] directory with yang files

In order to make URLs and produced content-types consistent, and to simplify the maintenance it makes sense to join these services into single one.

Due to restconf-nb service also serves yang resources of mounted devices it seems reasonable to move functionality  of yanglib to restconf-nb. 

In order to differentiate the sources it makes sense to use multiple schemas (and/or module-sets).
Suggested URL then became (host:port)/modules/(source-id)/(name)/(revision)
where source-id will reference where the yang resource expected to be taken from.

It requires clarification either yang resources from a local folder (current yanglib functionality) need to be populated to yang-library data or it can be omitted as not a part of global context (similar to resources on mounted device)