Can't force rebuilds of non-local packages

https://github.com/commercialhaskell/stack/issues/2805#issuecomment-263531315 reminded me of an issue I had recently but hadn't reported.

TL;DR. Sometimes you need to uninstall packages because they break somehow (because bugs) and you need to force a rebuild. I am 100% fine with "force a rebuild" if that's easier to implement. In fact, it'd be enough to extend `--force-dirty` (and its docs) to this use case.

```
$ stack install ghc-paths
Run from outside a project, using implicit global project config
Using resolver: lts-6.6 from implicit global project's config file: /Users/pgiarrusso/.stack/global/stack.yaml
$ stack install ghc-paths --force-dirty
Run from outside a project, using implicit global project config
Using resolver: lts-6.6 from implicit global project's config file: /Users/pgiarrusso/.stack/global/stack.yaml
$
```

Below I describe a crazy integration bug involving Homebrew, ghc-paths, and stack, as a concrete instance. This one might be Homebrew's fault but I'm not sure. I also realize I'm using a system GHC, but the plan is not to make that unsupported (since some users have valid use cases for that option and it makes sense to support them).

I use a system install of GHC 8.0.1, and installed it with Homebrew. Then, Homebrew updated it to version 8.0.1_1 (and then 8.0.1_2), which means some files were moved (they're now in `/usr/local/Cellar/ghc/8.0.1_2` rather than `/usr/local/Cellar/ghc/8.0.1`). But the GHC binary is still on the PATH, so 99% of packages don't care.

But I use intero. Which depends on ghc-paths. Which figures out those paths during the build, apparently—so they became invalid, and the Homebrew update broke intero.
So I had to uninstall ghc-paths to rebuild it. That was very challenging: I had to use `stack exec ghc-pkg unregister` (or some variation), but that wasn't enough because a new install fetched the compiled ghc-paths from some earlier snapshot. After a few tries, looking at verbose logs to figure out the involved earlier snapshot, I managed a clean reinstall. I sometimes removed the existing files by hand (after listing them with `stack exec ghc-pkg list`).

Yes, I know I could have nuked the entire cache, but many of you would probably find that annoying. Most users would only have that option, and I'm not 100% sure I don't have some silent corruption in the snapshot (though everything seems to work as expected).

I'm using stack 1.2.0 if that matters.