Support parallel builds with multiple projects using the same output directory

[lukecwik]

It is very convenient to have one offline repository that is at the root of the project instead of one per sub project. Currently the plugin fails because multiple sub-projects attempt to copy the same resource to the same directory and fail:

java.io.IOException: Failed to copy full contents from '~/.m2/repository/com/google/guava/guava/18.0/guava-18.0.pom' to 'offline-repository/com/google/guava/guava/18.0/guava-18.0.pom'

[realPyR3X]

I also have this issue. Currently I have to disable parallel when that task is executed.