Improve LabelSource parsing

[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   41.64m ± 3%   41.37m ± 3%       ~ (p=0.165 n=10)
CheckRules-4     8.531 ± 2%    8.544 ± 2%       ~ (p=0.971 n=10)
geomean         596.0m        594.5m       -0.25%

              │   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    967.0Mi ± 0%   966.8Mi ± 0%       ~ (p=0.684 n=10)
geomean         162.8Mi        162.8Mi       -0.01%

              │   old.txt   │              new.txt               │
              │  allocs/op  │  allocs/op   vs base               │
FindEntries-4   131.9k ± 0%   131.9k ± 0%       ~ (p=0.791 n=10)
CheckRules-4    14.10M ± 0%   14.10M ± 0%       ~ (p=0.436 n=10)
geomean         1.364M        1.364M       +0.00%

[codecov-commenter]

Codecov Report

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

Project coverage is 94.96%. Comparing base (3ad2373) to head (1887ad0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1181 +/- ## ========================================== + Coverage 94.91% 94.96% +0.05% ========================================== Files 104 104 Lines 12360 12499 +139 ========================================== + Hits 11731 11870 +139 Misses 480 480 Partials 149 149 ```

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