Complete class aliases whose targets do not exist

[Declspeck]

This commit adds support and a test for the following case:

use Some\Namespace\ClassWhichDoesNotExist;

ClassWhichDoesN| // Completes to ClassWhichDoesNotExist

This is primarily useful in cases where the target class is not indexed for some reason.

This PR is based on #599 so it cannot be merged before that and only the last commit f5b1256 contains new changes.

[codecov]

Codecov Report

Merging #605 into master will increase coverage by 0.34%.
The diff coverage is 90.71%.

@@             Coverage Diff              @@
##             master     #605      +/-   ##
============================================
+ Coverage     81.63%   81.98%   +0.34%     
- Complexity      910      947      +37     
============================================
  Files            61       61              
  Lines          2075     2231     +156     
============================================
+ Hits           1694     1829     +135     
- Misses          381      402      +21

Impacted Files	Coverage Δ	Complexity Δ
src/Server/TextDocument.php	75.55% <100%> (+0.18%)	57 <0> (+1)	⬆️
src/Index/AbstractAggregateIndex.php	86.04% <71.42%> (-7.44%)	26 <6> (ø)
src/Index/Index.php	77.41% <82.5%> (+7.6%)	54 <32> (+32)	⬆️
src/FqnUtilities.php	83.87% <90.47%> (+13.87%)	0 <0> (ø)	⬇️
src/CompletionProvider.php	94.34% <96.8%> (-0.17%)	112 <73> (+4)

[felixfbecker]

Is this still relevant?