Summary:
Populate the new environment variable if relevant path for it found.

For the OSS getdeps builds of things depending on fb303 this will allow them to be able to be able to find the fb303 thrift files from the declared fb303 dependency, rather than needing duplicates/stubs inside each package.

Also adds python as dependency to fbthrift manifest,  found it needed it when testing

Reviewed By: HarveyHunt

Differential Revision: D33771979

fbshipit-source-id: 9635221c0e6a2ac225d7d0babe76f1c919b583de