When setting up the isolated build environment, buildthings would look

up the .local-packages and replace any mentions of dependencies with

those paths.
The problem came from when one of those dependencies had its own

.local-packages. Those would also be found at that package's own build

time, but the resulting wheel used for the parent's isolated build

environment wouldn't contain those replaced dependencies. This would

cause the main build environment to fail, since it had no knowledge of

the paths.
Now, buildthings will recurse into .local-packages from any package

mentioned in a .local-packages and mark them as explicit build-time

dependencies. This differs from the normal approach of applying those to

a list of dependencies, since we don't have the ability to intercept and

normalize any dependencies' dependencies. So we add them to the

build-time dependencies to ensure they get installed and satisfy the

packages.
Package-time dependencies are not affected.

Unit tests pass.
Successfully installed Review Board 9, which had a local-packages

dependency chain looking like:

reviewboard -> Djblets, django_evolution
Djblets -> django_evolution, django-assert-queries

This was failing to install django-assert-queries prior to this

change. With the fix, it now finds and installs it in the build

environment successfully.