Duplicate functions across ament linters with different functionality about ament_lint

[dirk-thomas]

A few questions coming to my mind:

• How much of the actual implementation can be unified, e.g. for generating the result file?
• If they would be unified where should these shared functions live? Currently the linter packages don't have a common dependency.
• Is the extra effort of maintaining the additional API worth deduplicating some (fairly simple) code snippets?

[JShep1]

• I believe that all of the above functions can each be unified - the core functionality of each one is identical especially for get_files and find_executable. get_xunit_content would require certain code changes within each linter so that a standard way of formatting the report variable can be used.
• Possibly within a separate directory within ament_lint?
• Not sure that much maintenance other than what is already being done would be required. It could be argued that this new maintenance whether additional or not is outweighed by the ease of updating all linters xunit output in one swipe