Support targets with plugins

Plugins don't work yet:

```
$ hrepl //hrepl/tests:PluginLib
GHCi, version 8.6.5: http://www.haskell.org/ghc/  :? for help
ghc: can't find a package database at bazel-out/k8-fastbuild/bin/external/stackage/ghc-tcplugins-extra-0.3/_install/ghc-tcplugins-extra-0.3.conf.d
Error running GHCi.
```

See also `//hrepl/tests:plugins_test`.
