[23.0] Don't fail CWL tool parsing when Cheetah not installed

[mvdbeek]

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:

build util and tool-util packages, create new virtualenv, install both, then python -c "from galaxy.tool_util.parser import get_tool_source"

License

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