Rework contract init

[ifropc]

What

Rework contract init. Not it does:

1. When run on empty/non-existing directory: initialize cargo worspace and add hello-world contract there
2. When running on an existing project root: add a new hello-world contract package

Why

Changes according to https://github.com/stellar/stellar-cli/issues/1586

Known limitations

--overwrite support has been removed. If we decide to keep it in https://github.com/stellar/stellar-cli/pull/1628, this PR should return it back