Move `dotty.runtime` to `scala.runtime`

[nicolasstucki]

We will first need to copy the contents of dotty.runtime into scala.runtime and use them, then we will need to re-bootstrap thee compiler, and then we will be able to delete the old version in dotty.runtime.

[nicolasstucki]

It is enough to move to old sources to library/src-non-bootstrapped. We can delete them after we re-bootstrap on M3.