During the development of buildthings, one of the goals was to allow

editable builds to exclude certain dependencies. This was removed

accidentally during the move to build isolation dependency control.
This change brings that functionality back and improves it.
All build types can now specify their own lists of dependencies, which

default to the standard dependencies set. They can also specify their

own exclusions.
Exclusions, both on dependencies and isolations, are now normalized.

The name in an exclusion can now match against a name with a version

specifier in the dependencies list. Names are also canonicalized, so

they don't have to be an exact match.
Unit tests covering the dependency options for build isolation, package

metadata, and builds have been added, along with the underlying methods

used for normalizing and filtering dependencies. These tests have been

moved into buildthings/tests/ so that we can reliably import test

modules for utilities.

Built packages in Review Board using both dependencies and exclusions.

Checked the resulting lists.
Installed in editable mode and verified it also used the right

dependencies and exclusions.
All unit tests pass.