Fix API::tap_from_source_download for relative paths.

[reitermarkus]

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Extracted from #16689.

[Bo98]

Not entirely sure what would be passing relative paths here? Formulary should be called with an absolute path.

But OK code wise

[reitermarkus]

Not entirely sure what would be passing relative paths here?

Nothing yet, but the fix in #16689 uses this method to check if a path is a source download path, so it should work with any path and never raise an error.