pants: Add `system_user` detection to `pants-plugins/uses_services` (+ `mongo` detection improvements) #5259

Job	Run time
Skip Duplicate Jobs Pre Job	5s
Self-check on Python 3.8 - Python 3.8	7m 56s
Pack Tests - Python 3.8	1m 49s
Lint Checks (black, flake8, etc.) - Python 3.8	1m 59s
Integration Tests (chunk 1) - Python 3.8	3m 43s
Compile (pip deps, pylint, etc.) - Python 3.8	3m 2s
Integration Tests (chunk 2) - Python 3.8	5m 0s
Unit Tests (chunk 1) - Python 3.8	6m 10s
Pack Tests - Python 3.9	1m 49s
Lint Checks (black, flake8, etc.) - Python 3.9	2m 7s
Unit Tests (chunk 2) - Python 3.8	4m 36s
Integration Tests (chunk 1) - Python 3.9	3m 46s
Compile (pip deps, pylint, etc.) - Python 3.9	3m 20s
Unit Tests (chunk 1) - Python 3.9	6m 22s
Integration Tests (chunk 2) - Python 3.9	5m 0s
Unit Tests (chunk 2) - Python 3.9	4m 49s
Slack notification for failed master builds	3s
	1h 1m 36s

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