Completion for relative names

[Declspeck]

This pull request is based on #599 so if it needs changes, I will have to port them to this PR too. I might just rebase and force-push this, so don't get too attached to the branch (e.g. don't branch from this).

This PR adds support for completion for relative names, i.e. completions at namespace\| and namespace\Class\In\The\Current\Namesp|

[codecov]

Codecov Report

Merging #602 into master will increase coverage by 0.38%.
The diff coverage is 90.87%.

@@             Coverage Diff              @@
##             master     #602      +/-   ##
============================================
+ Coverage     81.63%   82.02%   +0.38%     
- Complexity      910      951      +41     
============================================
  Files            61       61              
  Lines          2075     2236     +161     
============================================
+ Hits           1694     1834     +140     
- 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	94.46% <96.89%> (-0.05%)	116 <77> (+8)

[felixfbecker]

Is this still relevant?