Provides means of querying information about build WAS Offer command to find base directory for package

This command, say `package-path`, would return the base directory (where the `.cabal` file lives) for a certain package, e.g.

```
$ cat stack.yaml
packages:
  - ./foo # Contains foo.cabal, package name foo
  - ./bar # Contains bar.cabal, package name bar
resolver: lts-3
$ stack package-path foo
./foo
```
