Fix Conduit 1.3 deprecation warnings
- The `Conduit`, `Producer` and `Consumer` aliases are deprecated and
`ConduitT` is used directly instead
- `=$=` is deprecated and replaced by `.|`
- Add LICENSE files for all packages.
- Add descriptions for packages that were missing one.
- Work around google/proto-lens#69 by symlinking third_party into
tensorflow-proto.
The tensorflow-records package implements encoding/decoding of the
format, and the tensorflow-records-conduit package provides wrappers and
utilities for use with Conduit.
