Lazily initialize AggregateSourceLocator to speedup bootstrapping

[staabm]

implement the idea described in #5538 (comment)

refs phpstan/phpstan#14072

=> 25% faster end-to-end analysis

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review with whitespaces ignored

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with foo.php

<?php

echo "";

before this PR

➜  phpstan-src git:(2.1.x) ✗ hyperfine 'php bin/phpstan analyze foo.php --debug -vvv'
Benchmark 1: php bin/phpstan analyze foo.php --debug -vvv
  Time (mean ± σ):     772.3 ms ±   9.8 ms    [User: 557.5 ms, System: 210.4 ms]
  Range (min … max):   758.7 ms … 787.8 ms    10 runs

after this PR

➜  phpstan-src git:(lazy-loca) ✗ hyperfine 'php bin/phpstan analyze foo.php --debug -vvv'
Benchmark 1: php bin/phpstan analyze foo.php --debug -vvv
  Time (mean ± σ):     552.9 ms ±   6.7 ms    [User: 441.0 ms, System: 109.2 ms]
  Range (min … max):   548.6 ms … 571.3 ms    10 runs

[ondrejmirtes]

/cc @xificurk Look at this 😊