Clear files from cached scanners in distribute()

[ryanohoro]

Describe the change

When pulling a scanner object from cache, distribute() clears the files property before use.

      name = scanner['name']
      und_name = inflection.underscore(name)
      scanner_import = f'strelka.scanners.{und_name}'
      module = importlib.import_module(scanner_import)
      if und_name not in self.scanner_cache:
          attr = getattr(module, name)(self.backend_cfg, self.coordinator)
          self.scanner_cache[und_name] = attr
      options = scanner.get('options', {})
      plugin = self.scanner_cache[und_name]

      # Clear cached scanner of files
      plugin.files = []

Fixes #292

Describe testing procedures

./strelka-oneshot -l - -f src/python/strelka/tests/fixtures/test.html | wc -l
2
./strelka-oneshot -l - -f src/python/strelka/tests/fixtures/test.html | wc -l
2
./strelka-oneshot -l - -f src/python/strelka/tests/fixtures/test.html | wc -l
2

Sample output

No changes

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings