Only choose buildable components for build-depends and build-tool-depends dependencies.

See https://github.com/haskell/cabal/pull/5304#issuecomment-388626764 and https://github.com/haskell/cabal/issues/3978#issuecomment-388504699 for an example involving `http-client-0.5.12.1`. The library stanza contains

```
-- See build failure at https://travis-ci.org/snoyberg/http-client/jobs/359573631
if impl(ghc < 7.10)
buildable: False
```

The solver doesn't backtrack to find an older version with a buildable library, and cabal fails with (from https://github.com/haskell/cabal/pull/5304#issuecomment-388626764):

```
$ cabal new-build -w ghc-7.8.4 --index-state='2018-05-09T06:20:23Z'
Error:
Dependency on unbuildable library from http-client
In the stanza 'library'
In the inplace package 'zzz-0'
```

This issue is a continuation of #3978, which described a bad error message for an unbuildable component. Its regression test tests the current error message:

https://github.com/haskell/cabal/blob/898fcc230fb537064f0992cfffb5955b6bc36d34/cabal-testsuite/PackageTests/NewBuild/T3978/cabal.out#L1-L6

This issue should probably be fixed as part of #4087.

Start by reproducing the reported command with GHC 7.8.4 and the stated index state, then read cabal-testsuite/PackageTests/NewBuild/T3978/cabal.out and the linked discussions for the existing behavior. Done means dependency solving selects a buildable component or an appropriate older version instead of failing on an unbuildable library, with the regression test updated accordingly.

Written by the indexing model from the issue text.