cmd/gopherbot: use longest prefix matching for compiler/runtime issue labeling

[wuyang9311]

When an issue is filed against a compiler/runtime-owned sub-package
that is not explicitly listed in the owners file (e.g.,
"cmd/compile/internal/noder"), gopherbot's labelCompilerRuntimeIssues
fails to apply the "compiler/runtime" label because it requires an
exact package name match.

This happens because dev.golang.org/owners has entries for
"cmd/compile" but not for every sub-package like
"cmd/compile/internal/noder". New sub-packages are added over time
and the owners file doesn't always keep up.

This change adds a longestPrefixMatch helper that falls back to
finding the longest prefix match in the crtPackages set when an
exact match is not found. For example, "cmd/compile/internal/noder"
will match "cmd/compile" as its closest compiler-owned ancestor.

The prefix boundary check (requiring a '/' after the candidate
prefix) ensures that "cmd/compilefoo" does not accidentally match
"cmd/compile".

Unit tests cover 27 cases including exact matches, prefix matches
at various depths, non-matches, longest-prefix preference, boundary
conditions, and empty crtPackages.

Fixes golang/go#80421

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[wuyang9311]

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