dream-lock: support multiple top-level packages

[DavHau]

This changes the structure of the dream-lock.json to support a list of top level packages instead of a single top level package.
Despite these changes, some parts of the framework still depend on defaultPackage not being null or unset.
That means there is still some work left to, but from here on we can smoothly transition away from enforcing a defaultPackage.

removed dream-lock fields:

• translator (currently covered by translatorParams)
• translatedBy (currently covered by translatorParams)
• mainPackageName
• mainPackageVersion

added dream-lock fields:

• defaultPackage (equivalent to old mainPackageName)
• packages

Other changes done on the rust subsytem:

• fix a bug in the rust translator where the path path-like sources was wrong whenever the path length is greater 1.
  (example ripgrep).
• process only one input directory (scenarios with several input dirs are not yet really supported by the framework and this will be refined through Architecture: Project Discovery #76 anyways)

@yusdacra Could you review the rust changes?

[yusdacra]

The Rust changes look fine to me! One minor thing, the example specification for Rust subsystem should be updated as well to remove the packages field.