Reduce build dependencies

[Rahix]

Due to the new build system as of #157, we have quite a lot of additional build-time dependencies. Among them are also quite a few that we don't really need, like clap, env_logger, url, and similar. These are pulled in as child-dependencies of libraries we use for code-generation.

Similar to what I have done in #200 / Rahix/atdf2svd#81 for atdf2svd, we should try making these dependencies optional upstream to make our build-process more lean. I don't think this has to happen right now, but we should strive to get rid of all the unnecessary bloat over time.

Cc: @LuigiPiucco

[LuigiPiucco]

From my inspection, the only crate that doesn't yet enable us to do this is svdtools, so I opened rust-embedded/svdtools#289. Once that's released, I have the changes to make dependencies optional ready locally, I'll open a PR.