Shows a descriptive error, doesn't suggest "add requires" quick fix, and excludes such modules from completion.

Review ID: IDEA-CR-28310

Reproducible only if 'Good code is red' inspection from DevKit is switched on

ReadOnly -> Unmodifiable; introduced UnmodifiableView distinguish between unmodifiable and unmodifiable view

This is a generalization of 41f7bbae, which was initially made only
for the HgTaskBranchesTest, while the issue can happen for any VCS test.

myCachedDistinctClassPairs now removed as moreless efficient implementation of DistinctPairSet#contains is possible

though no ambiguity is there by jls, users may prefer to specify qualifier to help understand the code (IDEA-159680)

and on real jdk (8 or 9 on TeamCity)

ref: CPP-8067 Allow opening single files in CLion