Resolve yang-parser-spi naming

yang-parser-spi currently holds content which is quite ungroomed. It has all sorts of things:

• org.opendaylight.yangtools.yang.parser.spi.meta.AbstractDeclaredStatement. This actually is yang-model-spi stuff and we have a replacement in yang-parser-rfc7950
• org.opendaylight.yangtools.yang.parser.spi.meta.CopyHistory. This is yang-parser-rfc7950-(api|spi) stuff, as it relates to the implementation.
• org.opendaylight.yangtools.yang.parser.spi.source.BelongsToModuleContext et al. are clearly YANG-specific and are clearly yang-parser-rfc7950-spi (i.e. exposed to plugins)
• parser.spi.validation – that is completely unclear, but definitely not visible to the causal user

Clean all this up, shifting it to new componenets. yang-parser-spi will probably end up being very thin with things relating to yang-parser-api implementation!