Add support for varied model conformance

https://tools.ietf.org/html/rfc7895#page-9 and https://tools.ietf.org/html/rfc7950#section-5.6 defines a 'conformance' leaf to be associated with a module advertised by the device.

There are two conformance levels:

• implement
• import

We currently always 'implement' a module included in CrossStatementSourceReactor, even if it is only needed to satisfy imports (e.g. is added as libSource).

For modules pulled in from the module library to satisfy imports we should mark the resulting effective module as import only and only process its reusable pieces (groupings, typedefs, identities).