Break out `javalib/` module from `scalalib/`

[lihaoyi]

Right now we kind of fake it with a javalib package in the scalalib module, but ideally we would have javalib be a separate module and have scalalib depend on it

[lefou]

Since separating javalib and scalaib it more to please our users but not so much a technical requirement (since we don't really have different dependencies, as these are all loaded dynamically), we could as well rename the current scalalib module to jvmlib and just shuffle the classes around, so we have proper packages mill.jvmlib (e.g. for RunModule), mill.javalib and mill.scalalib packages.