The old Djblets extension packaging code made a number of assumptions

about how extension static media would be packaged. It had some

capabilities to override settings (such as providing custom LessCSS

global variables), but made assumptions on the environmental setup and

the usage of setuptools.
This change redoes much of this, splitting

djblets.extensions.packaging into submodules for static_media and

setuptools_backend.
The static_media module contains two classes:


StaticMediaBuildContext: Manages state and general configuration for

  building static media.


StaticMediaBuilder: Sets up a Django and Pipeline environment and

  initiates the build process.


This can then be driven by a packaging backend. The setuptools_backend

module contains the original functions and classes comprising the

setuptools-based backend, though wrapping the new static_media

classes. Subclasses can set a custom class for either of the

static_media classes on the Setuptools backend in order to customize

behavior.
One of the features of the new backend is support for defining a

.npm-workspaces directory, which contains symlinks to other trees that

may provide static media files that may be needed by the build system.

This defaults to a djblets symlink, but subclasses can add additional

backends.
Down the road, the expectation is that we'll have better support for

using modern pyproject.toml-configured build backends for

extensions, and this change would help get us there.
The original classes can still be imported and used as before, but are

now deprecated, with useful information on moving to the modern classes.

Unit tests pass.
Tested building rbintegrations with this change, both with legacy

usage in Review Board and with updated usage.