[new-build] cabal.project: warn about flags for unknown package

I had a section

```
package wallet-new
profiling-detail: all-functions
```

in my `cabal.project` file, and I was scratching my head why it didn't seem to have any effect. Turns out that it should have been

```
package cardano-sl-wallet-new
profiling-detail: all-functions
```

where `cardano-sl-wallet-new` is the name of the cabal package (as opposed to `wallet-new` which was the name of the directory in which it was stored). That by itself is sensible enough but it would have been nice to get a warning "ignoring flags for unknown package `wallet-new`" or something along those lines.

Start by tracing how cabal.project package sections are parsed and how flags for named packages are applied. Reproduce the example with an unknown package name, then verify that the completed change reports a warning identifying the ignored package while valid package flags continue to work.

Written by the indexing model from the issue text.