UX improvement for typos in `allow-newer`

If I write this in my `cabal.project` file:

```
allow-newer:
http-api-data:base
, postgrseql-simple:base
```

then `cabal` doesn't complain, but it also tells me that the `base` version is Too New for `postgresql-simple`. It takes a minute to scan the text and realize my typo.

It'd be nice if `allow-newer` failed if the package name wasn't in the dependency set.

Start with the `allow-newer` entry in the `cabal.project` example and reproduce the behavior using `cabal` with the misspelled `postgrseql-simple` package. Trace how package names are checked against the dependency set; done means an unknown package name is reported instead of producing a misleading `base` version warning.

Written by the indexing model from the issue text.