Allow marking derivations as `contentAddressed`.
In that case the build outputs will be moved to paths determined by
their content (like `add-to-store`).

This support is still very basic: We can't atm depend on a ca
derivation, and even how to retrie the actual path of the outputs isn't
specified

That way we'll be able to find what an alias points to

Allows building dependencies of content-addressed derivations
(though not transitive deps because we don't alias the dependencies
correctly)

Remove the input drvs from the derivation before recomputing the output
hash because otherwise these prevent early cutoff from happening

Allows to indirectly depend on a CA derivation

Otherwise fetching some (but not all) outputs of a derivation from the
cache fails because we request the pathInfo for a non-existing store
path

If we don't do that, the paths won't be added to the sandbox, and so the
build will fail

When building with `--store`, we need to resolve the store paths to
their actual on-disk location before doing any concrete operation on
them

- Resolve aliases when computing the actual `bash` path
- Rewrite the derivation like we do for building