Zubin authored Dec 13, 2021 and Marge Bot committed Jan 11, 2022

This patch adds the ability to fetch and store dependencies needed for
boostrapping hadrian. By default the script will download the
dependencies from the network but some package managers disallow network
access so there are also options to build given a supplied tarball.

The -s option allos you to provide the tarball

    bootstrap.py -d plan-bootstrap-8.10.5.json -w /path/to-ghc -s sources-tarball.tar.gz

Which dependencies you need can be queried using the `list-sources` option.

    bootstrap.py list-sources -d plan-bootstrap-8.10.5.json

This produces `fetch_plan.json` which tells you where to get each source from.

You can instruct the script to create the tarball using the `fetch` option.

    bootstrap.py fetch -d plan-bootstrap-8.10.5.json -o sources-tarball.tar.gz

Together these commands mean you can build GHC without needing
cabal-install.

Fixes #17103