Reject output-only format_source references in the XML linter

[jmchilton]

Remove the XML linter exemption that treated an output name as a valid format_source reference.

At runtime, output formats can be derived from tool inputs and input collections, but not from another output. The schema likewise documents format_source as an input reference. Restricting the linter to collected input parameter paths makes linting consistent with those semantics.

Fixes #23449.

How to test the changes?

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. Lint a tool whose output uses another output's name as its format_source.
  2. Confirm that the linter reports the reference as an unknown parameter.

Existing linter suite: pytest test/unit/tool_util/test_tool_linters.py (104 passed).

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.