Rewrite boot in Python

One step closer to being able to drop the Windows Perl tarball. We
previously attempted to do this in D3567 but were forced to revert due
to Windows problems.

Acknowledgements:
 * @Phyx kindly contributed the codepath allowing this to work on
Windows.

Test Plan: Validate

Reviewers: hvr, austin, Phyx

Subscribers: erikd, thomie, rwbarton

Differential Revision: https://phabricator.haskell.org/D3918