backpack doesn't work with split-objs

```
mac:coda> cat cabal.project.local
split-objs: True
mac:coda> cabal new-build coda-algebra
In order, the following will be built (use -v for more details):
- coda-common-0.0.1 (lib) (first run)
- coda-group-relative-0.0.1 (lib) (first run)
- coda-group-relative-0.0.1 (lib with Delta=coda-common-0.0.1-inplace:Coda.Relative.Delta.Type) (first run)
- coda-algebra-0.0.1 (lib) (first run)
Configuring library for coda-common-0.0.1..
Preprocessing library for coda-common-0.0.1..
Building library for coda-common-0.0.1..

on the commandline: warning: ignoring -fsplit-objs
[ 1 of 12] Compiling Coda.Algebra.Zero ( Coda/Algebra/Zero.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Algebra/Zero.o )
[ 2 of 12] Compiling Coda.Relative.Delta.Type ( Coda/Relative/Delta/Type.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Relative/Delta/Type.o )
[ 3 of 12] Compiling Coda.FingerTree ( Coda/FingerTree.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/FingerTree.o )
[ 4 of 12] Compiling Coda.Algebra.Ordered ( Coda/Algebra/Ordered.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Algebra/Ordered.o )
[ 5 of 12] Compiling Coda.Syntax.FromText ( Coda/Syntax/FromText.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Syntax/FromText.o )
[ 6 of 12] Compiling Coda.Syntax.Alex ( Coda/Syntax/Alex.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Syntax/Alex.o )
[ 7 of 12] Compiling Coda.Syntax.Name ( Coda/Syntax/Name.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Syntax/Name.o )
[ 8 of 12] Compiling Coda.Syntax.Prefix ( Coda/Syntax/Prefix.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Syntax/Prefix.o )
[ 9 of 12] Compiling Coda.Util.Bits ( Coda/Util/Bits.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Util/Bits.o )
[10 of 12] Compiling Coda.Util.BitQueue ( Coda/Util/BitQueue.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Util/BitQueue.o )
[11 of 12] Compiling Paths_coda_common ( /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/autogen/Paths_coda_common.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Paths_coda_common.o )
[12 of 12] Compiling Coda.Version ( Coda/Version.hs, /Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Version.o )
/Users/ekmett/coda/dist-newstyle/build/x86_64-osx/ghc-8.2.1.20171030/coda-common-0.0.1/build/Coda/Algebra/Ordered_o_split: getDirectoryContents:openDirStream: does not exist (No such file or directory)cabal: Failed to build coda-common-0.0.1 (which is required by
coda-algebra-0.0.1).
```

Notice the

```
on the commandline: warning: ignoring -fsplit-objs
```

warning and the tail of the last line, which wrapped

```
getDirectoryContents:openDirStream: does not exist (No such file or directory)cabal: Failed to build coda-common-0.0.1 (which is required by coda-algebra-0.0.1).
```

It looks like ghc is telling cabal "stahp", but cabal just assumes that the split-objs are produced when it proceeds.