Milestone: split parser-impl into multiple artifacts

Current organization of parser-impl provides very little pressure to maintain proper inter-package dependencies, as the entire component is built in one go. Splitting the artifact will cause maven to detect circular dependencies, enforcing proper design.

We should introduce new artifacts as follows:
yang-parser-spi:
yang.parser.spi
yang.parser.spi.meta
yang.parser.spi.source
yang.parser.spi.validation
yang-parser-reactor (depending on yang-parser-spi):
yang.parser.stmt.reactor
yang-parser-rfc6020 (depending on yang-parser-reactor):
yang.parser.stmt.rfc6020
yang-parser-rfc7950 (depending on yang-parser-reactor, yang-parser-rfc6020):
yang.parser.stmt.rfc7950
yang-parser-impl (depending on all of the above):
the rest of the packages