Export build.sbt values inside sbt.

* Expose the values PAST the Eval/sbt.compiler package.
* Find projects using the name API rather than finding htem and dropping all values immediately.
* Adds a test to make sure the .sbt values are discovered and set-able
* Expose .sbt values in Set command and inside BuildUnit methods.
* Ensure `consoleProject` can see build.sbt values.
* Add notes for where we can look in the build if we want to expose .sbt values between files.
Fix the val extractor to only pull sbt types rather than all.
Minor cleanups and fix Eval tests to work correctly.
Merge pull request #1456 from sbt/wip/build-sbt-vals-in-set-command

Export build.sbt values inside sbt.