Make filesToProcess a local variable

### Affected version

head

### Bug description

The way it's set up now there's a bug in CpdReport and perhaps PmdReport where executeReport gets an empty filesToProcess if it's called without first calling canGenerateReport, since only the latter populates the map.

Locate CpdReport and PmdReport and trace executeReport and canGenerateReport to see how filesToProcess is populated. Verify the behavior when executeReport is called directly, and consider the issue resolved when both reports receive the expected files without requiring canGenerateReport first.

Written by the indexing model from the issue text.