hadrian: Add multi:<pkg> and multi targets for starting a multi-repl

This patch adds support to hadrian for starting a multi-repl containing
all the packages which stage0 can build. In particular, there is the new
user-facing command:

```
./hadrian/ghci-multi
```

which when executed will start a multi-repl containing the `ghc` package
and all it's dependencies.

This is implemented by two new hadrian targets:

```
./hadrian/build multi:<pkg>
```

Construct the arguments for a multi-repl session where the top-level
package is <pkg>. For example, `./hadrian/ghci-multi` is implemented
using `multi:ghc` target.

There is also the `multi` command which constructs a repl for everything
in stage0 which we can build.