wasm-compose: improve the tool to make configuration files optional

[peterhuene]

This PR improves wasm-compose such that it no longer requires a configuration file to be present to operate.

To support this change, the tool is now explicitly told which component is the root component to compose.

To resolve dependencies, a set of search paths are searched automatically for components satisfying the dependencies of the root (and transitively their dependencies as well).

Dependencies that are not found are kept as instance imports in the composed component (i.e. can now import host interfaces from the composed component).

The tool now only exports everything from the root component in the composed output.

A configuration may still be specified to have the same flexibility in specifying the complete or partial instantiation graph and where to explicitly locate dependencies.

Additionally, exported instances from an instance may now be easily aliased such that those exported instances may participate in the instantiation graph too.