Executable imposes a lower bound on `aeson` in Stackage

[jkachmar]

Just noticed this when going to build LTS-18.24; it seems like the http2 client executable is built by default, which means that it imposes a bound on aeson:

aeson-1.5.6.0 ([changelog](http://hackage.haskell.org/package/aeson-1.5.6.0/changelog)) (Adam Bergmark <[email protected]> @bergmark) is out of bounds for:
- [ ] http2-3.0.3 (>=2). Grandfathered dependencies. @kazu-yamamoto. Used by: executable

I think it might be possible to gate this behind a build flag that defaults to disabling the executable component (which would prevent Stackage from trying to construct a build plan). If that's reasonable I could try to open a PR with it.

[kazu-yamamoto]

To build client, you need to specify -f devel to cabal. But yes, the hpack command is the source of this issue. Do you want to try fixing this issue and send a PR? Or, should I just do it?