crate_universe: Use tera for generating top-level repository rules

[illicitonion]

Right now in crate_universe we do hand-crafted string templating for the http_archive/git_repository rules, and then invoke tera for the build_file_contents.

We shouldn't be doing hand-crafted string templating. We should also have some unit tests for "this crate turns into this output", rather than only testing generation with integration tests.

I'll put together a patch :)

[UebelAndre]

Is there not more that can be lifted from cargo-raze::rendering? I feel like this functionality is all there. Also, #657 seems relevant to this as well.

[illicitonion]

#655 covers all I'm thinking about doing for this issue :) very happy to re-use more of cargo-raze if we can, too

[illicitonion]

This got done :)