Create a "release" artifact that is configured to pull necessary jars from Ivy/Maven

[lihaoyi]

Currently, Mill depends on a number of compiler-bridge jars which are created as part of the Mill build. Both sbt scalaplugin/test and the executable generated by sbt scalaplugin/test:assembly reference these jars, by hard-coding their absolute paths on disk. Thus the executable built by sbt scalaplugin/test:assembly would only work on the same computer you ran the SBT command on. The number of locally-built jars that Mill needs to reference programmatically is likely to grow over time (e.g. TestRunner will likely move into it's own jar, along with the ScalaModule zinc integration)

Obviously, this will not do if we want to make Mill usable by anyone on the internet. What we need is a "release" build of the Mill executable that will resolve the necessary compiler-bridge (and other) jars from Maven Central (or from your on-disk publishLocal maven cache, for testing) instead of hard-coding absolute paths to recently-built jars on disk.

[lihaoyi]

(Until #5 lands, we can test this against jars publishLocaled by our SBT build)