rustpkg: Support arbitrary dependencies in the install API

api::install_pkg now accepts an argument that's a list of
(kind, path) dependency pairs. This allows custom package scripts to
declare C dependencies, as is demonstrated in
rustpkg::tests::test_c_dependency_ok.

Closes #6403
auto merge of #9654 : catamorphism/rust/rustpkg-c-dependencies, r=brson

r? @brson api::install_pkg now accepts an argument that's a list of
(kind, path) dependency pairs. This allows custom package scripts to
declare C dependencies, as is demonstrated in
rustpkg::tests::test_c_dependency_ok.

Closes #6403