Search compatible versions in fetching packages

Previous version of code reported endpoints that does not have pckMeta
(not downloaded) yet and does not have exactly equal targets as
incompatible. This caused some package to be downloaded multiple times
and caused conflict errors (#1147).

This patch adds proper code for searching compatible version in
currently fetching packages basing on targets.

It also simplifies check for case when we have resolvedVersion and
candidate's target is range.
Add some tests and documentation for #1310