Separate out YangIdentifierClassNamingStrategy

Separate out YangIdentifierClassNamingStrategy

Our current ClassNamingStrategies rely on the fact that each YANG
statement's argument provides an AbstractQName from which we can extract
a YANG identifier (i.e. AbstractQName.getLocalName()).

ietf-restconf:yang-data's argument is any non-empty String, so we just
cannot keep this contract.

Separate current handing into a separate class, which inherits the
notion of having a nodeIdentifier(), except that fact is purely internal
field. This also neatly re-joins Module and CamelCase to use the same
simpleClassName() method.

JIRA: MDSAL-675
Change-Id: Iaa1a43ba865ff78aa98ac69459e16373634eff40
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>