In this commit we instruct the build system to use the unified schema
files instead of collecting the `TUPLE-TREE-YAML` comments.
The fixes to C++ files are necessary because they depend on the order
of the fields in the schema, which was changed.
Replace the stub implementation of invalidation with the proper
implementation. A ReadPathCache is added to each global so that it can
keep tracks of what target are associated to which read paths.
Add the PipelineDescription object that describes the structure of a
pipeline. This will be used by all the downstream users instead of
pipeline accessor methods via PipelineC.
Alessandro Di Federico