rustpkg: Search for crates in the current directory

As per #8520, find crates in the current working directory even
if it's not a workspace.

Closes #8520
auto merge of #9736 : catamorphism/rust/rustpkg-issue-8520, r=cmr

r? @cmr As per #8520, find crates in the current working directory even
if it's not a workspace.

Closes #8520