Improve external links finder to not yield duplicate links.

--HG--
branch : distribute
extra : rebase_source : 78e932fca32ee0ee1f50794cf998f4e7db78131b
avoid naming problems

--HG--
branch : distribute
extra : rebase_source : 29eeb99013055b8d27cad7f7e8898d06a865b188
actually filter the links

--HG--
branch : distribute
extra : rebase_source : cb6e3497e1f8594181f10110cbc833bd6c81f89e
Added Dirley to contributors

--HG--
branch : distribute
Add unique_everseen from Python 2.7 docs

--HG--
branch : distribute
Use a wrapper to ensure unique values on find_external_links. Factors out uniqueness test into a re-usable decorator and simplifies the body of find_external_links.

--HG--
branch : distribute
First round of prepping setuptools for inclusion in Python 2.5: move
site.py to setuptools/site-patch.py; reinstate 'python -m easy_install'
support; use distutils' "upload" command when running under 2.5.

--HG--
branch : distribute
extra : source : fbb6c89a74aa47a556a936202d2e50051b067940
extra : histedit_source : 778122fc56bcca6f5a1dbd5a475df11d29028730
Update changelog

--HG--
branch : distribute
Added tag 0.6.42 for changeset f30167716b65

--HG--
branch : distribute
Merge with distribute 0.6.42