upgrade to sbt 1.3.7 (from 1.2.8)

in order to also allow building on JDK 13+, we also disambiguate some
overloads and remove a no-longer-necesary scalaVersion override (at
the build definition level)

note that to use current sbt, we must also use current mima
and exclude some spurious issues it reports

this commit also includes some Dotty changes, contributed by
Guillaume Martres. he describes them as follows:

Fix stdlib compilation with dotty when using sbt 1.3

When we compile the standard library with dotty, we actually compile the
scala-library and dotty-library sources together in one go since we
can't in general reuse a dotty-library compiled with an old
scala-library. There were two problems with the way this was done that
were somehow masked so far (likely because of classpath pollution, but I
don't know what changed in sbt 1.3 exactly):

- Compiling the stdlib requires having all sources on the -sourcepath,
  but the sources from dotty-library itself were missing.
- the files in scalaShadowing/ in dotty-library do need to be compiled,
  the comment above the exclusion was wrong.

Additionally, I also removed the other exclusions done to files in
dotty-library because they're no longer necessary.

Co-authored-by: Guillaume Martres <[email protected]>

Author: 2 people