Improve label source discovery

[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   39.74m ± 1%   40.27m ± 1%  +1.33% (p=0.015 n=10)
CheckRules-4     8.525 ± 2%    8.526 ± 2%       ~ (p=0.971 n=10)
geomean         582.0m        585.9m       +0.67%

              │   old.txt    │               new.txt               │
              │     B/op     │     B/op      vs base               │
FindEntries-4   27.41Mi ± 0%   27.41Mi ± 0%       ~ (p=0.089 n=10)
CheckRules-4    976.8Mi ± 0%   974.9Mi ± 0%  -0.19% (p=0.019 n=10)
geomean         163.6Mi        163.5Mi       -0.10%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   131.9k ± 0%   131.9k ± 0%       ~ (p=0.079 n=10)
CheckRules-4    14.19M ± 0%   14.17M ± 0%  -0.08% (p=0.004 n=10)
geomean         1.368M        1.367M       -0.04%

[codecov-commenter]

Codecov Report

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

Project coverage is 94.78%. Comparing base (5205bcc) to head (1230821).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1161 +/- ## ======================================= Coverage 94.78% 94.78% ======================================= Files 104 104 Lines 12258 12258 ======================================= Hits 11619 11619 Misses 487 487 Partials 152 152 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.