Add support for Python #source-db internal flavor

Created by: bigfootjon

This adds support for running pyre in buck mode.

See the cherry-picked commit messages in this PR for more information.

Test plan:

ant
bin/buck build //programs:gen_buck_info#source-db
alias buck=`realpath bin/buck`
cd $personal_project
buck build $personal_target#source-db
buck run $personal_target -- --help
pyre

All of the above exit 0!