Move scalaJSExternalCompileSettings outside the sbt plugin.

It has been relevant only to the build of Scala.js itself for
a few months, now.
Fix #102: test:packageJS and compile:packageJS have the same outputs.

Now compile:packageJS emits X-extdeps.js, X-intdeps.js and X.js,
whereas test:packageJS emits X-test-extdeps.js, X-test-intdeps.js
and X-test.js.

In the process, made these three destinations configurable through
settings
artifactPath in packageSomethingJS
and
moduleName in packageSomethingJS