Buildthings injected dependencies into Setuptools through a bit of an

annoying workaround that we inherited from the in-tree build backends

for Djblets and Review Board. This required telling Setuptools where to

find a package-requirements.txt file, which we'd then write to before

Setuptools got a chance to run, and that it should look up dependencies

dynamically.
This change swaps out that hacky workaround for another hacky

workaround. Instead of making the pyproject.toml file work around

this, we now do it inside of buildthings.
When setuptools loads pyproject.toml, it creates a _ConfigExpander

object that builds a finalized dictionary of settings (factoring in any

dynamic dependencies or other variables).
We now hook into this to add mutation functions for the final

dictionary. We remove any dynamic dependency configuration and then set

our final list of dependencies for the package. This is equivalent to

temporarily replacing the pyproject.toml file itself. This resulting

list then gets set in the internal Distribution objects for the

package definition.
While monkey-patching Setuptools is indeed hacky and could break, we are

at least pinning the version so there are no surprises. Setuptools

itself is full of patches like this on top of distutils to control

behavior.

Tested this on a package with a pre-set list of static dependencies

and a package with a list of dynamic dependencies. Built both and

inspected their resulting wheels for the dependencies lists. Verified

the expected final set of dependencies were present.