Merge workspace settings with CLI settings (#3045)
## Summary
This PR adds the structs and logic necessary to respect settings from
the workspace. It's a ton of code, but it's mostly mechanical. And,
believe it or not, I pulled out a few refactors in advance to trim down
the code and complexity.
The highlights are:
- All CLI arguments are now `Option`, so that we can detect whether they
were provided (i.e., we can't let Clap fill in the defaults).
- We now have a `*Settings` struct for each command, which merges the
CLI and workspace options (e.g., `PipCompileSettings`).
I've only implemented `PipCompileSettings` for now. If approved, I'll
implement the others prior to merging, but it's very mechanical and I
both didn't want to do the conversion prior to receiving feedback _and_
realized it would make the PR harder to review.
