Review Board's packaging requires a local build of Djblets, as it makes

use of metadata and packaging functionality that Djblets provides. We

often need to test Djblets changes with Review Board, so being able to

install against a local editable Djblets package is important, and for

this reason, we turned off build isolation.
Unfortunately, this in turn broke Haystack installation. Haystack

doesn't provide wheels, just a tarball, and this tarball uses

setuptools-scm as part of the build process. What setuptools-scm

does is look for a local tag or branch that indicates the package

version, and falls back to a default of 0.0.0 if not found.
When not run in build isolation mode, the packaging in the tarball ends

up looking at Review Board's tree, computes 0.0.0 as the version, and

this fails to satisfy the version range pip is considering, which then

fails installation.
This means we need build isolation, but we also need local Djblets.
We're now trying a new approach. We now generate a temporary

requirements.txt that includes:

The Djblets source tree as an editable (if found as editable already)
dev-requirements.txt
doc-requirements.txt, if needed

We then pass that to pip, in the default build isolation mode. This

allows pip to consider all the dependencies across all these at once.

It also ensures that a local editable of Djblets will be used as

editable. It does still need to install its dependencies in the build

isolation environment, but as this generally uses cached wheels, it's

pretty fast.
The result should be the same as before, but without the side effects

Haystack suffers from and without pulling down a published Djblets.

Created new virtual environments and installed in the following

conditions:

With a local Djblets in editable mode.
With a parent environment install of published Djblets.
Without any existing install of Djblets.

Verified that in the first case, the existing Djblets tree is

used as part of package building with minimal setup time, and

that in the other cases a published Djblets is used instead.