Improve performance of `SourceMapper` #3109

Job	Run time
Dependency Validation	12s
Coding Guidelines	40s
Static Analysis	32s
Unit Tests (ubuntu-latest, 8.2)	10s
Unit Tests (ubuntu-latest, 8.3)	15s
Unit Tests (ubuntu-latest, 8.4)	18s
Unit Tests (ubuntu-latest, 8.5)	11s
Unit Tests (windows-latest, 8.2)	36s
Unit Tests (windows-latest, 8.3)	34s
Unit Tests (windows-latest, 8.4)	40s
Unit Tests (windows-latest, 8.5)	38s
End-to-End Tests (ubuntu-latest, 8.2)	45s
End-to-End Tests (ubuntu-latest, 8.3)	41s
End-to-End Tests (ubuntu-latest, 8.4)	54s
End-to-End Tests (ubuntu-latest, 8.5)	45s
End-to-End Tests (windows-latest, 8.2)	1m 52s
End-to-End Tests (windows-latest, 8.3)	1m 57s
End-to-End Tests (windows-latest, 8.4)	1m 55s
End-to-End Tests (windows-latest, 8.5)	1m 58s
Build PHAR	39s
Code Coverage	2m 40s
Test PHAR (8.2, pcov)	18s
Test PHAR (8.2, xdebug)	35s
Test PHAR (8.3, pcov)	39s
Test PHAR (8.3, xdebug)	35s
Test PHAR (8.4, pcov)	21s
Test PHAR (8.4, xdebug)	33s
Test PHAR (8.5, pcov)	25s
Test PHAR (8.5, xdebug)	30s
	22m 48s

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