Convert BUILD.$name-$version.bazel from tera to serde_starlark

[dtolnay]

Context: see #1734.

This PR converts all of crate_build_file.j2, partials/crate/*.j2 and partials/starlark/*.j2 to serde_starlark.

Remarks

• I kept the output of bazel run //crate_universe/3rdparty:crates_vendor out of this PR to keep GitHub's code review interface usable. You can find the output generated code in 44c264a.

• I tried to keep the conversion 1-to-1 to the largest extent possible. As an example, build_script.j2 duplicated most of the content of common_attrs.j2 even though it probably could have used that template via include, and I have mirrored the same here by not using #[serde(flatten)] common: CommonAttrs inside of CargoBuildScript. There is probably plenty of opportunity for cleaning up things in followup PRs.

• I came across several bugs in the existing code. Again, I mostly stayed away from these to keep the PR bounded in scope. For example the fact that rustc_flags is stored as a BTreeSet<String> is not correct; compiler flags are ordered and sorting them is not correct. For example -C debuginfo=2...

  rules_rust/crate_universe/src/context/crate_context.rs

  Line 178 in 532e60f

  pub rustc_flags: SelectStringList,

• One bug that I fixed because in the new implementation it was easier to fix than to not fix, is that all existing uses of selectable_list.j2 and selectable_dict.j2 were broken because they all had the same failure mode as crate_universe does not consolidate configurations that match the same set of platforms. #1417. The generated code using selects.with_or could end up generating a map with duplicate keys if multiple distinct Cargo cfg expressions mapped to the same subset of supported platform triples. My implementation follows the same approach as Fix issue #1417 - Group deps by platform triple rather than cfg(...) string. #1647, but that PR only applied it to deps and aliases, not e.g. rustc_env and everything else.

• Anecdotally the new implementation has strong "if it compiles, it works" property. I mostly typed out the implementation of this PR top to bottom without running crates_vendor, and as soon as the whole thing was written and the compiler was okay with it, it was generating the output I wanted. I have not done much work in the templates in the past but I do not imagine they have this property.

Remaining work

• select_with_or in private/selects.bzl is no longer used by the code generator's output. I had to keep it in this PR because all the generated BUILD files still include calls to it. It can be deleted after all those are regenerated.

• One of the tera templates still uses the old serialization of SelectList so the starlark serialization for it isn't a normal Serialize impl but instead an associated function. This causes there to be a bunch of serialize_with = "SelectList::serialize_starlark" attributes needed. This can be cleaned up after the last tera template is converted.

• A lot of the contents of crate::context can be cleaned up after they're no longer used by tera. Tera needs those data structures to be serializable, whereas with serde_starlark we're not serializing anything from crate::context, only from crate::utils::starlark. For now I've just added a bunch of serde(skip) attributes in crate::context on all the parts tera is no longer using. Edit: apparently we still need to serialize all the fields in Context so that they end up in cargo-bazel-lock.json.

• There are 2 remaining tera templates after this PR, module_bzl.j2 and vendor_module.j2.

[illicitonion]

This looks great, thanks for putting it together! Feel free to update the vendored files (as a separate commit in this PR)

[dtolnay]

I made 2 force pushes:

• first, purely a rebase of the already reviewed code to fix merge conflict with Re-pinned all dependencies managed by crate_universe #1735.
  https://github.com/bazelbuild/rules_rust/compare/5a39df2dc26e6bbb6805fffb742b6c218bc0d130..d5a493e47989ad51cc38445c4b0500dbfc3257fd

• second a small change to address Convert BUILD.$name-$version.bazel from tera to serde_starlark #1743 (comment) to replace FIXME -> TODO.
  https://github.com/bazelbuild/rules_rust/compare/d5a493e47989ad51cc38445c4b0500dbfc3257fd..bba5b8b0550988056d6867be5274ee9d9023a97b

[dtolnay]

• Rebased to resolve conflict with Add gen_binaries annotation to control which bins to make target for #1718

[illicitonion]

Ship it! Thanks!

[dtolnay]

Clean rebase.