Allow Blueprint build statements and rules to reference rules and pools
defined outside of Blueprint.
Change-Id: I48644497d16a4c2bab88db400dbe7f7fe169d933
Turn PackageContext into an interface so that build systems can wrap it
to add more custom helpers.
This does introduce an API change, though it should be fairly simple.
NewPackageContext used to provide an opaque *PackageContext struct, now it
provides a PackageContext interface.
Change-Id: I383c64a303d857ef5e0dec86ad77f791ba4c9639
Make integrating with go tools easier by putting the blueprint package
files in the top level directory of the git project instead of in a
subdirectory called blueprint.
Change-Id: I35c144c5fe7ddf34e478d0c47c50b2f6c92c2a03
