Specifically, when the Scala version for sbt is the same as that for the project being built,
the jars in UpdateReport should be the same as those in ScalaProvider. This is because the
loader will come from the ScalaProvider, which uses jars in the boot directory instead of the
cache. The first part of the fix for #661 checks that loaded classes come from the classpath
and so they need to line up.
Introduce a way to configure incremental compiler itself instead
of underlying Java/Scala compiler.
Specific list of changes in this commit:
* Add a method to `xsbti.compile.Setup` that returns incremental
compiler options as a `java.util.Map<String, String>`. We considered
statis interface instead of a `Map` but based on mailing
list feedback we decided that it's not the best way to go because
static interface is hard to evolve it by adding new options.
* Since passing `java.util.Map<String, String>` not very convenient
we convert it immediately to `sbt.inc.IncOptions`
* Add options argument to various methods/classes that implement
incremental compilation so in the end options reach
`sbt.inc.IncOptions` object
* Add `incOptions` task that allows users to configure incremental
compiler options in their build files. Default implementation of
that tasks returns just `IncOptions.DEFAULT`
* Both system property `xsbt.inc.debug` and `IncOptions.relationsDebug`
trigger debugging of relations now. In the near future, we should
deprecate use of `xsbt.inc.debug`.