scripted test that reproduces the issue with deletes
fixing issues with deletion in sbt-protobuf:

Before this change, deletions were not handled correctly by the PB
plugin. If a schema file was removed, the corresponding java file was
not. Moreover, if all schema files were removed, protoc was invoked
with no source files, causing a build error. Both of these conditions
can lead to bad builds and require clean builds in order to produce
correct results.