WIP NX-685: Improve "incomplete" document detection

*WIP: DO NOT MERGE YET*

Instead of testing for "latest", "stable", etc just
accept the fact that versions map is Map<String, String>
and in that case create incomplete documents for all entries

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 fetched from registry to
have the non-incomplete version.

Issue
https://issues.sonatype.org/browse/NX-685
Removed this change, no more need for existence of dist-tags and no more any "specially handled" tag.