rustpkg: Test that different copies of the same package ID can exist in multiple workspaces

The test checks that rustpkg uses the first one, rather than complaining
about multiple matches.

Closes #7241
auto merge of #8712 : catamorphism/rust/rustpkg-issue-7241, r=pcwalton

r? @brson ...multiple workspaces

The test checks that rustpkg uses the first one, rather than complaining
about multiple matches.

Closes #7241