Duplicate functions across ament linters with different functionality

Many of the linters (`clang-format`, `cppcheck`, `cpplint`, `flake8`, `pep257`, etc.) utilize identically named functions (`get_xunit_content`, `get_files`, and `find_executable`) which all express a similar but slightly different functionality based on the linter being used.

### Core Functionality of Identically-Named Functions
Note: Given this issue, some of the received arguments may vary between linters
1. `get_xunit_content`

- Receive a report consisting of logging data, a test name, and an elapsed time
- Log to an `.xml` file

2. `get_files`
- Receive the paths to consider and the desired file extensions
- Return the valid files

3. `find_executable`
- Receive a file name indicating the desired executable and additional specified paths (default is None)
- Search PATH and additional paths for the executable

These functions could likely be unified and condensed.