simplify jinja templates codegen

[OmarTawfik]

I Needed the ability to reference templates from other crates in a later PR downstream, so I'm doing the refactoring in a separate PR (this one) to make it easy to review:

• parallelize all rendering of all templates in the codebase in one pass, instead of per-crate.
• allow importing any templates with relative path from REPO_ROOT

As a nice side effect, infra check codegen (that runs on every file save) is now down to ~2s from ~6s.

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[teofr]

LGTM, thanks!