Improve performance of `SourceMapper` #3091

Job	Run time
Coding Guidelines	44s
Dependency Validation	14s
Static Analysis	1m 15s
Unit Tests (ubuntu-latest, 8.3)	20s
Unit Tests (ubuntu-latest, 8.4)	20s
Unit Tests (ubuntu-latest, 8.5)	15s
Unit Tests (windows-latest, 8.3)	37s
Unit Tests (windows-latest, 8.4)	36s
Unit Tests (windows-latest, 8.5)	37s
End-to-End Tests (ubuntu-latest, 8.3)	47s
End-to-End Tests (ubuntu-latest, 8.4)	1m 22s
End-to-End Tests (ubuntu-latest, 8.5)	45s
End-to-End Tests (windows-latest, 8.3)	1m 52s
End-to-End Tests (windows-latest, 8.4)	2m 1s
End-to-End Tests (windows-latest, 8.5)	1m 53s
Code Coverage	2m 32s
Build PHAR	39s
Test PHAR (8.3, pcov)	20s
Test PHAR (8.3, xdebug)	38s
Test PHAR (8.4, pcov)	18s
Test PHAR (8.4, xdebug)	36s
Test PHAR (8.5, pcov)	25s
Test PHAR (8.5, xdebug)	30s
	19m 36s

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