Duplicated plugins if they are present in more than 1 location

This happens in very particular conditions.

How to reproduce:
  - install rapache 0.7 from the PPA
  - enter a standalone rapache dir (like a branch downloaded with bzr)
  - start the branched rapache
  - open the virtualhost editor

Expected result:
  - you see the plugins of the branched rapache, plus the ones in your homedir

What happens:
  - you'll see both the plugins in the PPA install and the plugins in the branched directory

Explanation:
plugins are read from every search path, thus rapache reads the PPA ones and the local directory install. It is unable to discern duplicates and give priority to any of the duplicated plugins and avoid loading the other one.

Why is this bad:
if a user downloads a newer plugin version in his home directory he could see the plugin duplicated instead to have the newer plugin to replace its duplicates

What to do:
assign a plugin priority based on location (to be discussed) and load only the right plugins.

This affects 0.7 but shuold be take care of in the next release I think.