Refactor utils.LabelsSource

[github-actions[bot]]

Benchmark diff:

goos: linux
goarch: amd64
pkg: github.com/cloudflare/pint/cmd/pint
cpu: AMD EPYC 7763 64-Core Processor                
              │   old.txt   │              new.txt               │
              │   sec/op    │   sec/op     vs base               │
FindEntries-4   39.86m ± 3%   40.35m ± 1%       ~ (p=0.123 n=10)
CheckRules-4     8.530 ± 2%    8.512 ± 2%       ~ (p=0.631 n=10)
geomean         583.1m        586.0m       +0.51%

              │   old.txt    │               new.txt               │
              │     B/op     │     B/op      vs base               │
FindEntries-4   27.41Mi ± 0%   27.41Mi ± 0%       ~ (p=0.796 n=10)
CheckRules-4    971.2Mi ± 0%   972.8Mi ± 0%  +0.16% (p=0.005 n=10)
geomean         163.2Mi        163.3Mi       +0.08%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   131.9k ± 0%   131.9k ± 0%       ~ (p=0.984 n=10)
CheckRules-4    14.12M ± 0%   14.13M ± 0%  +0.07% (p=0.004 n=10)
geomean         1.364M        1.365M       +0.04%

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.91%. Comparing base (7d2a022) to head (4b70d4a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1180 +/- ## ======================================= Coverage 94.91% 94.91% ======================================= Files 104 104 Lines 12358 12360 +2 ======================================= + Hits 11729 11731 +2 Misses 480 480 Partials 149 149 ```

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