Handle multiple extracted versions of same webjar

[mkurz]

Those webjars get extracted in subfolders.

See lengthy comment in plugins.sbt of this pull request.

• Same as in Handle multiple extracted versions of same webjar sbt-coffeescript#33

Update:
Also see

• Pulling in same WebJar with same version should be disallowed and handled as conflict webjars/webjars-locator-core#141 (specially "Side note on creating subfolders")