Template Haskell changes to names and package keys

[Edward Z. Yang]

> Being able to get all the packages that have a particular name sounds
> good!  Note that if "-hide-package" / "-hide-all-packages" is used, I
> wouldn't want those packages in the results.

This is not entirely clear; you could imagine a package also coming with
an 'exposed' bit, so you could filter for exposed/not exposed pacakges.

> Like Dan, I'm a little less keen on #1.  Here's why:
> 
> * PkgName is in the internal "Language.Haskell.TH.Syntax" module
> 
> * It isn't actually documented what it means, so it's meaning can be freely
> bent.  There's no guarantee that you should be able to generate them.
> 
> * I couldn't find any examples of its usage that would be broken by this
> new semantics.  I've done a search on github[1] to find usages of PkgName,
> and I only found one use[2] that uses PkgName which would be affected by
> this change.  This example is in the TH lib itself, and so would hopefully
> be fixed by this change.
> 
> On the other hand, since it's rarely used, such an API breakage wouldn't be
> that impactful, so it's not that big of a deal.

Actually, this proposal was prompted by two separate reports of
breakage:

    https://github.com/ekmett/lens/issues/496
    https://ghc.haskell.org/trac/ghc/ticket/10279

So, like it or not, people seem to be depending on this API, which means
it's worth at least discussing a little when we change it :)

Cheers,
Edward