Only add CopyFilesFromBuild if it is used
Fix broken vcs dispatch

It was broken by a bad translation from given/when/default to
for/if/elsif/else in b7924a9.
Actually use the "allow_dirty" option

Previously setting the option had no effect.
v0.132960

    - Actually use the "allow_dirty" option instead of ignoring the
      user-set value and always using the default.
    - Fix broken vcs dispatch (broken since v0.132800)
    - Only load CopyFilesFromBuild plugin if it is used