Proto files can include other proto files.
E.g. nanopb comes with a `nanopb.proto` include that contains some
nanopb-specific extensions.
This patch makes it possible to include this file as well as specifying
additional proto include folders.
I added the include to `simple.proto` so it gets exercised by the test.
Prepare for handling pkg state with files. So it requires having the
path defined before declaring targets. In addition, it cleans up the
old git-download target.
