DEP_FOO_KEY-like system that can work without "links"

Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#any-build-script-metadata

Implementation: https://github.com/rust-lang/cargo/pull/16436

Would it make sense to allow usage of DEP_<name>_<key environment variables in a build.rs for dependencies that do not link to native libraries?

Context: I would like to allow crates like this one to expose Cap'n Proto schema definitions, and in particular to allow Cap'n Proto's import mechanism to work across crate boundaries. (See https://github.com/dwrensha/capnpc-rust/issues/30.)

For example, say I write a my_app crate that depends on the sandstorm crate, and I want my_app to have some capnp schemas that import from the sandstorm schemas. For this to work, the my_app build.rs needs to know where to find the source code of the sandstorm schemas. One way to accomplish that would be for the sandstorm crate's build.rs to write its current working directory to a DEP_SANDSTORM_IMPORT environment variable, through the mechanism described here. However, that only works if the sandstorm crate has a "links" field in its Cargo.toml. Since the sandstorm crate does not actually need to link to a native library, adding such a field feels superfluous and possibly prone to mysterious bugs.

Start with the unstable Cargo build-script metadata documentation and the implementation discussed in Cargo PR 16436. Trace how DEP__ is currently exposed only through the links mechanism; done would mean dependencies without native-library links can expose the metadata needed for cross-crate Cap’n Proto imports.