Render project .vm overrides with Velocity when useProjectFiles is set (#267)

[elharo]

Fixes #267

Problem

When useProjectFiles is enabled, a resource override placed in the project
root is copied verbatim via FilteringUtils.copyFile, bypassing Velocity
processing. Unlike copyResourceIfExists, copyProjectRootIfExists also only
looked for <name>, never for the <name>.vm template variant, so .vm
overrides were silently ignored and the bundle template was used instead.

Changes

• copyProjectRootIfExists now mirrors copyResourceIfExists:
  • checks for both <name> and <name>.vm in the project root
  • renders <name>.vm through Velocity (velocity.evaluate) with the bundle's
    source encoding, matching the behavior of bundle-resource templates
  • plain files are still copied verbatim

Tests

• New test testUseProjectFilesRendersVelocityTemplate: writes a local
  <basedir>/FILTER.txt.vm containing $project.name, sets useProjectFiles
  and asserts the output FILTER.txt contains the rendered project name.
• Verified the test fails without the fix and passes with it; full mvn verify
  (incl. spotless/checkstyle/RAT) passes.