Use Wasm to hash dep components, rather than `tar`

Refs #24

Instead of `tar`-packing the dependencies to compute the digest, use https://github.com/bytecodealliance/wasm-tools to assemble a Wasm containing all the interfaces and worlds within the fetched package and hash that.
For a given dependency `X`, produce `deps/X.wasm` alongside `deps/X` directory for backwards compatibility at least until the released version of https://github.com/bytecodealliance/wit-bindgen can take Wasm as input

- Given the state of tooling today, it appears that WIT package->Wasm direction is covered, what about Wasm->(expanded) WIT package?