Add a command to list available build targets

Given that `cabal.project` can list additional local components, each with its own cabal file, it is not always clear what the appropriate build targets are.

So perhaps add a command that will list the targets, e.g.

```
cabal list-targets

component Cabal
cabal (lib)
cabal:unit-tests (test-suite)
cabal:parser-tests (test-suite)
cabal:parser-hackage-tests (test-suite)
...
component cabal-install
cabal-install:cabal (exe)
cabal-install:unit-tests (test-suite)
,,,
```