pyoxidizer: allow specifying an existing project crate

[dae]

While building directly with cargo is possible (#467), a lot of the convenience that pyoxidizer provides is lost - you need to construct rather long command lines, and manually bundle files up separately (#466). What if instead of an all-or-nothing approach, pyoxidizer provided limited support for building from an existing Rust crate?

With this PR, the user can build a project created with init-rust-project with a single short command line:

$ PYOXIDIZER_CRATE_FOLDER=$(pwd) pyoxidizer build

It also happens to be somewhat faster, as crates like jemalloc don't get recompiled each time.

It would come with caveats:

• The user would be responsible for ensuring their crate sources are appropriate for the pyoxidizer version they are invoking
• It may not work with more elaborate embedding use cases - but then, the user is still free to drop back to raw cargo.
• I didn't encounter it in testing, but perhaps there might be cases where a stale build product could require the user to manually clear out their build folder?

What do you think?