Refactor directory walk into an iterator pattern

[svasista-ms]

Refactor opportunity:
The logic of reading directories, identifying cargo projects by Cargo.toml, figuring out if there is atleast one valid rust project or action failure etc could be abstracted into a custom Iterator (E.g: CargoProjectIterator) which we could be used in clean and build actions. This improves consistency across build and clean actions right now and any actions we may add later.

Originally posted by @krishnakumar4a4 in #638