CONFLICTS checks during the wrong stage

[Dominic Fandrey]

It appears CONFLICTS is checked before build instead of checking
it before install. Why?

# portmaster -Do lang/perl5.10 lang/perl5.8

===>>> Currently installed version: perl-threaded-5.8.9_3
===>>> Port directory: /usr/ports/lang/perl5.10
===>>> Launching 'make checksum' for lang/perl5.10 in background
===>>> Gathering dependency list for lang/perl5.10 from ports
===>>> No dependencies for lang/perl5.10

===>>> Starting build for lang/perl5.10 <<<===

===>>> All dependencies are up to date

===>  Cleaning for perl-threaded-5.10.1


===>  perl-threaded-5.10.1 conflicts with installed package(s): 
      perl-threaded-5.8.9_3

      They install files into the same place.
      Please remove them first with pkg_delete(1).
*** Error code 1

Stop in /usr/ports/lang/perl5.10.

===>>> make failed for lang/perl5.10
===>>> Aborting update