Mark 'ghc-boot-th' as non-installable.

[23Skidoo]

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[angerman]

Ah! I didn't even know we had this! Would this mean, that if I pushed flags through all dependencies, cabal would never try to rebuild those packages? (E.g. related to #4939).

Coincidentally there was a discussion on #hackage today with @sclv and @hvr where the question cam up if ghc --info should just list the non-reinstallable packages.

GHC basically knows about

wiredInUnitIds :: [UnitId]
wiredInUnitIds = [ primUnitId,
                       integerUnitId,
                       baseUnitId,
                       rtsUnitId,
                       thUnitId,
                       thisGhcUnitId,
                       dphSeqUnitId,
                       dphParUnitId ]

as being wired in packages.

Anyway, this is only tangential to this PR.

[23Skidoo]

@angerman There's --allow-boot-library-installs that allows you to ignore the blacklist.

[23Skidoo]

I discussed this with @hvr and we decided that it's better to keep the blacklist as small as possible. ghc-boot-th is only non-reinstallable because template-haskell depends on it, otherwise it could very well be.

[23Skidoo]

I wouldn't mind if the blacklist was populated automatically.

dphSeqUnitId,
dphParUnitId ]

Looks like those are not built by default, e.g. ghc-pkg list on my system doesn't list them.

[angerman]

I think dph is effectively dead