[Feature] ability to install rocks for which there exists only a `.src.rock` entry in the manifest

[mrcjkb]

In #291, if there is no rockspec entry, but there is a src.rock entry in the manifest, rocks will try to install from the rockspec's source first, falling back to fetching the src.rock if the rockspec.source can't be fetched.

This is fine, but it has one edge case: If the rockspec source can be fetched, the lockfile is populated with the rockspec.source hash and not the .src.rock hash. To be able to fetch the rockspec from the src.rock, in a pure manner, nix will need to know the hash of the .src.rock.

To solve this, we should:

• Split build into separate pub(crate) fetch_source and build steps (we can still expose the current build function).
• Build using the src.rock if no rockspec entry exists in the manifest.