Add `cabal packages`

I would like to propose that cabal learns about `packages` or `new-packages`, as a wrapper around `ghc-pkg`. The motivation here, is that one needs to use only a single command, and not need to know about `ghc-pkg`.

I'd like to see:
```
cabal packages [--global|--local]
```

where
```
cabal packages --global
```
would be
```
ghc-pkg list --package-db=$HOME/.cabal/store/ghc-VERSION/package.db
```
and
```
cabal packages --local
```
would be
```
ghc-pkg list --package-db=dist-newstyle/packagedb
```

I'm not sure if no flag should be permitted, and which of both scenarios it would be, or if it should list global AND local.

---
Slightly unrelated, but why do we have `package.db` for global and `packagedb` for local?