Dotty version should not be hardcoded in dependencies

[nicolasstucki]

The dotty version should not be hardcoded. I think this should work:

"ch.epfl.lamp" %% "dotty" % scalaVersion.value,

Originally posted by @allanrenucci in #6017

[nicolasstucki]

This should be fixed also in https://github.com/lampepfl/dotty-quoted.g8/blob/master/src/main/g8/build.sbt#L13

[smarter]

Use "dotty-compiler" instead of "dotty", the latter is a meta-project that we should stop publishing eventually (we still need it because I hardcoded it into sbt, but I fixed this for sbt >= 1.3)