Really look for packages/XXX/.../YYY/$name.$version/opam

Which new means that XXX and YYY can contain '.'. Also, if a package appear multiple times in the repository (because of symlinks for instance) that should just pick the latest it see while it is scanning the tree.
Compiler descriptions can be in subdirectories as well.

compilers/XXX/.../YYY/xxx.{comp,descr}

This means that we can now start sorting the 'official' compiler against the other ones.

Fix #483
Add tags to compiler descriptions