Add support for verifying repository/dependency state when uploading packages

[anevilyak]

Currently, when adding/updating a package to the HaikuPorts repository via the build system, the commit hook only verifies that the modified package was in fact uploaded. It would be nice if it could also verify that the updated package's dependencies are fully satisfied, and/or that it doesn't break any existing dependencies. This would ensure that the repository is always in a consistent state.

From discussing it with Oliver, this could potentially be implemented by extending package_repo itself to do the verification, or alternatively the build system could do so by collecting the packages to be uploaded, and using that as an additional temporary repository when verifying dependencies.