NX-685: Improve "incomplete" document detection

Instead of special handling of tags "latest", "stable", etc just
accept the fact that versions map might be Map of String-String
and _in that case_ create incomplete documents for all entries.

Same for shrinking: instead of expecting the existence of dist-tags
and one single tag (which is not even required to exist, or, users
might define any other "own" special tags) just shrink the complete
versions map making it versionString-tagString map (or version
where tag for given version does not exists).

This fixes the NX-685, as it was caused by the problem that
document was not detected as incomplete, hence, on package
root fetch the document was not _refetched_ from registry (as incomplete
documents should be) to get the non-incomplete version.

Issue
https://issues.sonatype.org/browse/NX-685
Code changed and there is no more hard requirement (existence of dist-tags and existence of latest tag within). Basically, now the code will work with any document that passes validation OK and should work without exception.