Document parser-impl package structure

Package split of the parser implementation is inconsistent, with classes creeping into wrong packages and causing inter-package cycles.

One of the causes for this is the lack of details about what should individual packages contain.

Add pkg-info.java items with javadocs describing following packages:
parser.spi
parser.spi.meta
parser.spi.source
parser.spi.validation
parser.stmt.reactor
parser.stmt.rfc6020