rustpkg: Support sub-package-IDs

r? @brson Package IDs can now refer to a subdirectory of a particular source
tree, and not just a top-level directory with a src/ directory as its
parent.

For example, referring to the package ID a/b/c/d , in workspace W,
if W/src/a is a package, will build the sources for the package in
a/b/c/d (and not other crates in W/src/a).

Closes #6408