Add a new "dependency type" (PyPI) when parsing dependency definitions.

We had split up init from init_py3 because mach had
traditionally been invoked by either Python 2 or Python 3, and
the two couldn't share the same virtualenv.

Now that the same context isn't shared by both Python 2 and 3
3
(developers always use Python 3, and the remaining Python 2
usages are CI jobs that never reuse the objdir with Python 3),
We can centralize on a single default virtualenv.

I've called this "common" instead of "init" to clarify its
existing position as the virtualenv that's used by many different
commands. As we associate virtualenvs with requirement definitions,
it'll also make the file less confusing: it's a "common" requirement
definition as opposed to an "init" one.

This was originally set up so that tests wouldn't "create a new
virtualenv for no reason." However, virtual environments now will have
different packages installed, and therefore the separation is necessary.

So, for the virtual environment used for builds (regular or for tests):

• We want it to be able to reuse the build venv, if it already exists.
• We don't want to pollute a pytest virtualenv with build-specific
  packages.

Depends on D115635

Accidentally missing a return in a code path could mean that
handle_package(...) would accidentally do an action and
raise the "Unknown action" error.

Additionally, it simplifies the code a bit.

Depends on D115922

Child handle_package(...) invocations don't need to modify
sitecustomize.py, so don't pass it to them.

Depends on D115923

The pypi action uses pip to fetch a package and its dependencies

Depends on D115924

https://hg.mozilla.org/mozilla-central/rev/e1cd8757104d
https://hg.mozilla.org/mozilla-central/rev/2e37a3ce90c2
https://hg.mozilla.org/mozilla-central/rev/965304eb5799
https://hg.mozilla.org/mozilla-central/rev/7e8e5f4c8e73

Last year, we stopped vendoring Python packages that have native
code. Since we have only had pure-python packages since, the
Windows-specific qualifier (or excluder in the case of !windows)
hasn't been needed.

I don't foresee us need it again, but if anything we can peel it
back from hg history if this assumption is incorrect.

The mach_bootstrap:search_path() implementation is out of
date compared to virtualenv.py.

Since python2:, python3: and optional: packages are no
longer valid virtualenv requirement actions, they can be removed.

This has two benefits:

1. handle_package() becomes more clear - rather than referring to
   action and package with array index numbers, we now give
   them real names. The benefit here is also shown in up_to_date().
2. This makes the top-level parser for packages() less opinionated
   about sub-formats: if an action has a nested structure, it should
   have the flexibility to define what it looks like.

Depends on D117468

Having separate <name>.pth files in the virtual environments
isn't providing an advantage. We can simplify configuration
by putting all pth adjustments into a single file: mach.pth.

Depends on D117708

The mozbase modules were being unconditionally added to the
sys.path regardless of the Mach command being run, so there isn't
much value keeping them in a separate file. Besides, all other
source module paths are described in common_virtualenv_packages,
why is mozbase special?

In the future, we're going to want to make improvements here (such as:
there's a difference between informing mach of first-party code
versus defining which third_party vendored packages should be in scope,
and that workflow difference should be represented in-code).
It's useful to peel out the existing, less useful abstraction before
we can build a stronger one.

Depends on D115925

Check that all expected pypi packages are installed
when checking if a virtualenv is up-to-date.

Depends on D117711

Specifying a filename for sys.path modifications is
no longer necessary - if a path needs to be on the
Python sys.path, then just mark it with pth: for
it to go into a .pth file in the virtualenv.

https://hg.mozilla.org/mozilla-central/rev/4a326b86ad24
https://hg.mozilla.org/mozilla-central/rev/a705e6a13d96

https://hg.mozilla.org/mozilla-central/rev/f439c1345fd7
https://hg.mozilla.org/mozilla-central/rev/030a489e51f5
https://hg.mozilla.org/mozilla-central/rev/258461c83794
https://hg.mozilla.org/mozilla-central/rev/6a5c661cafe7
https://hg.mozilla.org/mozilla-central/rev/06df3dc28c58

Depends on D122893

https://hg.mozilla.org/mozilla-central/rev/f49440e40bfd