Cleanup yang-model-util

yang-model-util contains a number of classes relevant to implentations of yang-model-api as well as a number of classes relevant to users of yang-model-api.

The problem here is that the latter cannot be really tested with real models, as yang-model-util cannot depend on yang-parser-rfc7950 (as it depends on it).

Split out the implementation classes to yang-model-spi, which is used by yang-parser-rfc7950 (and others) and make yang-model-util depend on the parser for testing purposes.