PMD has been updated to use the renamed BaseTool class and to take

advantage of the new dependency management, to simplify code.
As part of this, configuration has become standardized, allowing the

pmd tool path to be located on the fileystem (if in $PATH) or

configured in the worker's exe_paths setting. This has the additional

small benefit of caching the pmd binary path, so it doesn't have to be

looked up in $PATH at execution time.
The old pmd_path setting has been deprecated. It will be transitioned

over when loading the configuration. Documentation has been updated to

discuss this.
The list of file extensions are now more forgiving. Extra commas, spaces,

and leading . for file extensions are handled correctly.
The documentation has been fleshed out to go over the fields present

during tool configuration.
Unit tests have been added for handle_file(), to give us a base for

future work and to ensure that the cached pmd path is being used.

Unit tests pass.
Tested dependency checking in the worker, with pmd in the $PATH

and with an explicit path provided in exe_paths.
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