Summary:
From the outset, we wanted to be sure that getdeps was able
to source and build the dependencies so that we knew that we'd have
a repeatable build.  This came at the cost of build times: having
to build boost on each CI run is a bit of a chore.

This commit adds three new elements to the manifest files:

* `rpms` - a list of RPM names that are all required to be present
  in order to consider the dependency satisfied
* `debs` - like `rpms` above, but scoped to debian package names
* `preinstalled.env` - a list of environment variables that if they
  are all set and non-empty will satisfy the dependency.

A new `--allow-system-packages` option to getdeps enables the new
logic that looks for system packages; it is off by default, but
enabled in the generated GitHub Actions workflows.

A new `install-system-deps` subcommand is provided that will attempt
to install the system packages needed to satisfy the build.  This
typically needs to be run via sudo and is thus broken out separately
from the main getdeps build flow.

I made a pass over the manifest files and added package names that
satisfy the build on ubuntu-18 and fedora-31.

shri-khare: I renamed the `Python3.7.6` manifest to just `python` as
part of this change; the version of python that it pulls in through
the normal build is the same and I believe that an equal or newer
version of python3 is available in the GH actions builder.

The `preinstalled.env` is used only by the boost manifest: it references
the name of an environment variable that is set by the github
windows hosts and that points to a pre-built and pre-installed
copy of boost.  Since there is no package manager that we can
easily query for this sort of thing, probing from the environment
seems like a reasonable and fast way to check for this.  We
may need to evolve this over time to become more feature rich,
but this seems like a good starting point.

This commit has the potential to save 20 minutes of build time
from each public CI build just due to the boost dependency alone!

Refs: https://github.com/facebook/watchman/pull/797

Reviewed By: yfeldblum

Differential Revision: D20740410

fbshipit-source-id: 6c38019449c54465127656c3d18a6ff1f30adaea