Ensure uniqueness of precomp dependences.
Ensure uniqueness of precomp dependences.

Some background:
I noticed duplicate entries appearing in some of the `lib/.precomp/*.rev-deps` when doing the following:
```
$ panda installdeps PDF
$ panda look PDF
.panda-work/1452532671_1$ time perl6 -I lib -e'use PDF::DAO::Doc'
.panda-work/1452532671_1$ time perl6 -I lib -e'use PDF::DAO::Doc'
```
With this patch, duplicates are removed from `*.rev-deps` and the second `use PDF::DAO::Doc` only takes about half the time.
jonathanstowe/rakudo,nbrown/rakudo,sjn/rakudo,lucasbuchala/rakudo,Gnouc/rakudo,samcv/rakudo,tony-o/rakudo,b2gills/rakudo,LLFourn/rakudo,dankogai/rakudo,ugexe/rakudo,salortiz/rakudo,MasterDuke17/rakudo,ugexe/rakudo,softmoth/rakudo,rakudo/rakudo,salortiz/rakudo,LLFourn/rakudo,tony-o/rakudo,labster/rakudo,zostay/rakudo,MasterDuke17/rakudo,awwaiid/rakudo,labster/rakudo,MasterDuke17/rakudo,lucasbuchala/rakudo,tony-o/rakudo,rakudo/rakudo,ab5tract/rakudo,nbrown/rakudo,azawawi/rakudo,awwaiid/rakudo,samcv/rakudo,usev6/rakudo,LLFourn/rakudo,softmoth/rakudo,salortiz/rakudo,rakudo/rakudo,sjn/rakudo,rakudo/rakudo,labster/rakudo,softmoth/rakudo,labster/rakudo,softmoth/rakudo,ab5tract/rakudo,zostay/rakudo,azawawi/rakudo,azawawi/rakudo,Gnouc/rakudo,Gnouc/rakudo,awwaiid/rakudo,b2gills/rakudo,salortiz/rakudo,nbrown/rakudo,dankogai/rakudo,labster/rakudo,Gnouc/rakudo,usev6/rakudo,MasterDuke17/rakudo,LLFourn/rakudo,salortiz/rakudo,dankogai/rakudo,lucasbuchala/rakudo,rakudo/rakudo,sjn/rakudo,tbrowder/rakudo,samcv/rakudo,raydiak/rakudo,awwaiid/rakudo,jonathanstowe/rakudo,samcv/rakudo,softmoth/rakudo,usev6/rakudo,tbrowder/rakudo,jonathanstowe/rakudo,jonathanstowe/rakudo,raydiak/rakudo,lucasbuchala/rakudo,zostay/rakudo,MasterDuke17/rakudo,salortiz/rakudo,ugexe/rakudo,MasterDuke17/rakudo,jonathanstowe/rakudo,azawawi/rakudo,awwaiid/rakudo,tbrowder/rakudo,tbrowder/rakudo,Gnouc/rakudo,nbrown/rakudo,zostay/rakudo,azawawi/rakudo,tony-o/rakudo,tbrowder/rakudo,dankogai/rakudo,ab5tract/rakudo,usev6/rakudo,tony-o/rakudo,labster/rakudo,b2gills/rakudo,sjn/rakudo,nbrown/rakudo,b2gills/rakudo,b2gills/rakudo,tony-o/rakudo,ugexe/rakudo,ab5tract/rakudo,rakudo/rakudo,lucasbuchala/rakudo,LLFourn/rakudo,raydiak/rakudo,usev6/rakudo,raydiak/rakudo,dankogai/rakudo,samcv/rakudo,sjn/rakudo,ab5tract/rakudo,ugexe/rakudo,tbrowder/rakudo,Gnouc/rakudo,nbrown/rakudo