new-update should deal with missing .cabal files

`new-update` has got better at working in more situations recently, but on HEAD it's still failing if `cabal.project` points to packages with missing `.cabal` files. `new-update` has no need for package metadata: the only project metadata it really cares about are `remote-repo` stanzas in `cabal.project` and closely allied files; if those files parse fine, it should do no extra validation on the existence/well-formedness of the `.cabal` files.

The use-case for this is a project where some of the `.cabal` files are generated by a tool contained within the project.

Start at the `new-update` entry point and reproduce the failure with a `cabal.project` that points to packages with missing `.cabal` files. Check that valid `remote-repo` stanzas in `cabal.project` and closely allied files are still handled, while missing or malformed package metadata is not validated.

Written by the indexing model from the issue text.