rust-build-system: Unvendor *-sys libraries in phase?

[John Soo]

Hi guix,

After working on a few rust packages, it looks like there could be another step 
on the process.  There are a number of libraries in the crates registry that 
wrap and vendor c libraries - libgit2, openssl, or jemalloc for example. The 
wrapping libraries, for reference, are usually called <c-library-name>-sys - 
libgit2-sys for example.

In the current rust-build-system, the onus of removing the vendored code falls 
on the ultimate consumer of the library. So, for instance, cbindgen would have 
to define a phase to remove the vendored code in a *-sys library even if that 
library is a transitive dependency.

What would you all think about adding a phase to the standard phases of the 
rust-build-system? I’m imagining some phase that would delete the vendored code 
and performed the other necessary steps to use the c libraries in /gnu/store.

Wdyt?

John