support first party pytest plugins; such as through install_from_resolve

[cburroughs]

Is your feature request related to a problem? Please describe.

There are a variety of ways to write and configure pytest plugins. I'd like to have first party (aka in my monorepo) plugins and configure them just as easily as third party ones (which use install_from_resolve / requirements).

That would mean some way to attach first party code to the pytest_runner.pex

Describe the solution you'd like

Naively I thought that this would "just work already"

[pytest]
install_from_resolve = "python-default"
requirements = [
   "//:reqs#pytest",
   "//my:plugin"
]

But that does not cause //my:plugin to appear in the sandbox. The contents of requirements is already a list of targets, so supporting first party targets either there or in a new field seems reasonable.

Describe alternatives you've considered

In this particularity case it is possible to configure the plugin through conftest.py. This works, but has downsides:

• conftest.py is spooky hard to reason about magic
• The plugin (and transitive deps) are duplicated as deps of every test instead of being part of the runner pex.

Additional context

pytest has a particularly rich plugin ecosystem, but this seems like a generally useful facility for any of the Python install_from_resolve subsystems.