Autocomplete complete used functions and constants

[Declspeck]

This add completion for

use function SomeNamespace\some_function;
use const SomeNamespace\SOME_CONST;

some_fun| // Completes to some_function
SOME_CONS| // Completes to SOME_CONST

This is based on #605 (Note - not the speedup PR, but another continuation of it), so this cannot be merged before #605 and #599 are merged.

[codecov]

Codecov Report

Merging #606 into master will increase coverage by 0.35%.
The diff coverage is 90.79%.

@@             Coverage Diff              @@
##             master     #606      +/-   ##
============================================
+ Coverage     81.63%   81.99%   +0.35%     
- Complexity      910      948      +38     
============================================
  Files            61       61              
  Lines          2075     2233     +158     
============================================
+ Hits           1694     1831     +137     
- 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	94.39% <96.85%> (-0.12%)	113 <74> (+5)

[felixfbecker]

Is this still relevant?