new-configure: Select the same plan subset as new-build.

[23Skidoo]

So that exes and other unnecessary components in dependencies are ignored.

Before:

$ cabal new-configure
Resolving dependencies...
In order, the following would be built (use -v for more details):
 - Crypto-4.2.5.1 (exe:SymmetricTest) (requires build)
 - Crypto-4.2.5.1 (exe:WordListTest) (requires build)
 - Crypto-4.2.5.1 (exe:SHA1Test) (requires build)
 - Crypto-4.2.5.1 (exe:QuickTest) (requires build)
 - Crypto-4.2.5.1 (exe:RSATest) (requires build)
 - Crypto-4.2.5.1 (exe:HMACTest) (requires build)
 - HaXml-1.25.3 (exe:Xtract) (requires build)
 - HaXml-1.25.3 (exe:Validate) (requires build)
 - HaXml-1.25.3 (exe:MkOneOf) (requires build)
 - HaXml-1.25.3 (exe:CanonicaliseLazy) (requires build)
 - HaXml-1.25.3 (exe:Canonicalise) (requires build)
 - HaXml-1.25.3 (exe:XsdToHaskell) (requires build)
 - HaXml-1.25.3 (exe:DtdToHaskell) (requires build)
 - HaXml-1.25.3 (exe:FpMLToHaskell) (requires build)
 - aeson-pretty-0.8.1 (exe:aeson-pretty) (requires build)
 - cpphs-1.20.2 (exe:cpphs) (requires build)
 - hS3-0.5.9 (exe:hs3) (requires build)
 - yaml-0.8.18 (exe:json2yaml) (requires build)
 - yaml-0.8.18 (exe:yaml2json) (requires build)

$ cabal new-build
Resolving dependencies...
Up to date

After:

$ cabal new-configure
Resolving dependencies...
Up to date

$ cabal new-build
Resolving dependencies...
Up to date

[mention-bot]

@23Skidoo, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dcoutts and @ezyang to be potential reviewers.

[ezyang]

OK SGTM.