Extend source file cache invalidation granularity to vanilla .scala files

[lolgab]

Mill's current cache has cache invalidation which has build file granularity.
If you have two files build.sc and other.sc and build.sc imports other.sc, changes in build.sc don't invalidate caches for targets defined in other.sc.
This works only with .sc since they explicitly state dependencies via import $file imports.
It would be nice to extend this to vanilla Scala files.
acyclic can track dependencies between vanilla Scala files. We could use something like it to implement the same support we currently have for *.sc files to *.scala files in the mill-build/src directory.

[lihaoyi]

Superseded by #2417, which implements fine grained method-level code change invalidation that works the same for .sc files and .scala files alike (and .java files too)