Move dotty compiler code to compiler.internal

This commit moves all code that was in dotty.tools into dotty.compiler.internal. The idea of a "tools" package seems to
be a holdover from "com.sun.tools.javac". This naming style is not in line with how packages are normally named in the
standard Scala distribution, hence the change. Note that I've done my best to update tests to make them compile/run, but
some tests that did not compile before still don't compile with this commit. These tests are not in the set run by "sbt
test", so I believe them to be stale (in fact, they should likely be removed in a future commit).