RewriteEngine on

# http://mentors.debian.net/contrib/test_1.0-1.dsc
RewriteRule ^(main|contrib|non-free)/([^_\.])([^_\.]+)_(.*)$ http://mentors.debian.net/debian/pool/$1/$2/$2$3/$2${3}_$4 [R=301,L]

# http://mentors.debian.net/test_1.0-1.dsc (it must be in main)
RewriteRule ^([^_\.])([^_\.]+)_([^\.])\.dsc$ http://mentors.debian.net/debian/pool/main/$1/$1$2/$1${2}_$3 [R=301,L]