irrelevant package

Dear GHC developers,

I am testing  packages  in  ghc-6.4.
I set 
      docon.cabal,  

which declares dependence on `data':     build-depends: data
The attempt to build  docon  by

  cd $doconSource
  runhaskell Setup.hs configure --ghc --prefix=$doconSource/inst
  runhaskell Setup.hs build

produces

  configuring ...
  compiling ...
  ...
  DPrelude.hs:1:0:
    Module `DPrelude' is a member of package dumatel-1.2.6.4.
    To compile this module, please use -ignore-package dumatel-1.2.6.4.


These does exist a package  dumatel-1.2.6.4,  and both the above 
packages have  DPrelude  among the module names. 
But why does Cabal look into irrelevant package? The dependence is 
set: only `data'.  

ghc-pkg -l  shows

  /usr/lib/ghc-6.4/package.conf:
    rts-1.0, base-1.0, haskell98-1.0, template-haskell-1.0, unix-1.0,
    Cabal-1.0, parsec-1.0, haskell-src-1.0, network-1.0,
    QuickCheck-1.0, HUnit-1.1, mtl-1.0, fgl-5.2, X11-1.1, HGL-3.1,
    stm-1.0, readline-1.0, (lang-1.0), (concurrent-1.0), (posix-1.0),
    (util-1.0), (data-1.0), (text-1.0), (net-1.0), (hssource-1.0)
  /home/mechvel/.ghc/i386-linux-6.4/package.conf:
    dumatel-1.2.6.4


The package specification is

----------------------------------------------------------
name:            docon
version:         2.9
license-file:    "../../license.txt"
copyright:       Serge Mechveliani (2005)
author:          Serge Mechveliani
maintainer:      mechvel@xxxxxxxx
homepage:        http://www.botik.ru/pub/local/Mechveliani/docon/
package-url:     http://www.botik.ru/pub/local/Mechveliani/docon/
                 http://www.haskell.org/docon/distrib/
synopsis:        computer algebra program written in Haskell
tested-with:     GHC
build-depends:   data
exposed-modules:
        DExport DPrelude Categs SetGroup RingModule DPair Z Fraction
        VecMatr LinAlg Pol GBasis Residue Permut Partition AlgSymmF
        .
other-modules:
  Iparse_ OpTab_
  Categs_ DPair_ Matr0_ Ring0_ Semigr_ Vec1_ Char_ Fract_ Matr1_
  Ring1_ Set_ Vec_ Common_ List_ Prelude_ Ring_ Vec0_
  Det_ Stairc_ Todiag_
  IdealSyz_ ResEuc0_ ResEuc_ ResRing__ RsePol0_ QuotGr_ ResEuc1_
  ResPol_ ResRing_ RsePol_ EPol0_
 Pgcd_ Pol_ RPol0_ EPol1_ FreeMonoid Pol1_ PolNF_ RPol_ EPol_
  GBasEuc_ Pol2_ Polrel_ FAA0_ GBasFld_ Pol3_ PP_ UPol0_ FAANF_
  GBas_ Pol__ RdLatP_ UPol_
  Pfact0_ Pfact1_ Pfact2_ Pfact3_ Pfact__ Pfact_
  HookBand_ Partit_ SymmFn0_ SymmFn1_ SymmFn_ Sympol_
  .
ghc-options:
  -fglasgow-exts -fallow-overlapping-instances
  -fallow-undecidable-instances
  -fno-warn-overlapping-patterns -fwarn-unused-binds
  -fwarn-unused-matches -fwarn-unused-imports
-----------------------------------------------------------



With kind regards,

-----------------
Serge Mechveliani
mechvel@xxxxxxxx