integration tests: Moves test_build's assertion code for better readability and fail output.
Moves most of di to a separate package, "fang".

fang will be published separately once all this work-in-progress is finalised.
Adds various notes about dependency management.

No functional changes, just comments.
Adds new config module to contain various config values.

These will probably be restructured so rather than being constants, they're values which are configurable via CLI options, etc.

This initial version is just trying to capture all the various "constants" build and link depend on.