The is_cached method now correctly handles store paths that have line
numbers appended (e.g., /nix/store/file.nix:123:456). Previously, these
paths would fail the existence check because the exact path with line
numbers doesn't exist as a file.
The fix adds a helper method that:
- First checks if the exact path exists
- If not, and the path contains colons, validates that the suffix
consists only of numbers (line:column format)
- If valid, strips the line numbers and checks the base file path
This ensures that cached references to specific file locations are
properly validated while avoiding false positives with files that
have colons in their names.
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