cabal install --constraint ... should warn or fail if non-existent package is named

Not a bug, but a suggestion: This command will happily progress
```
cabal build --constraint "this-package-does-not-exist==0"
```
I think it should warn loudly, or fail altogether. Motivation: I did have a typo in the package name that I wanted to constrain, and it took me a while to figure this out.
```
cabal --version
cabal-install version 3.6.2.0
compiled using version 3.6.2.0 of the Cabal library
```

Start by reproducing the reported behavior with cabal build --constraint "this-package-does-not-exist==0" using the stated cabal-install version. Trace how cabal-install handles --constraint and decide whether the expected result is a warning or failure; done means nonexistent package names are no longer silently accepted, with coverage for the reported typo case.

Written by the indexing model from the issue text.