Dependencies aren't loaded by default

`hrepl` doesn't expose dependencies by default. To load a dependency into the
interpreter, it needs to be either passed explicitly on the command line, or else
exposed with `:set -package ...`. This could be considered "working as intended"
since it's how the actual Bazel build rules work. However, it's not a great user experience:
- The `...` may be a mangled GHC package name
- `:set -package` causes all modules to reload and makes you lose all bindinsg.

The current recommended approach is to use `--package //some:label` which will add the given target as an exposed (compiled) dependency. But in all cases, if you want to expose
a new dependency you effectively need to start a new session.

Google's internal version of hrepl exposes everything, but that has its own tradeoff: Two
modules from different targets could conflict, and distinguishing between them with
`-XPackageImports` is cumbersome due to the mangled package names. Internally, we
modified GHC to be more permissive and allow unmangled labels as package names.