rustpkg: Begin allowing package scripts to call the default build logic

rustpkg/api.rs provides functions intended for package scripts to call.
It will probably need more functionality added to it later, but this is
a start.

Added a test case checking that a package script can use the API.

Closes #6401
auto merge of #7403 : catamorphism/rust/package-scripts, r=brson

r? @brson (or @graydon if available) rustpkg/api.rs provides functions intended for package scripts to call.
It will probably need more functionality added to it later, but this is
a start.

Added a test case checking that a package script can use the API.

Closes #6401