app: allow plugin inside first-level directories of plug-ins folders.

We don't search recursively but only at the first level. If a plugin is
in its own subdirectory, the entry point has to be named the same (minus
extension) as the directory. For instance my-plugin/my-plugin for a
binary, or my-plugin/my-plugin.(py|exe|…).
This way, a plugin can load shared objects (libraries, other script
files, etc.) without polluting the main plug-ins directories, and in
particular without interfering with other plug-ins.
This is a first step to fix bug 757057 (DLL files which were used in
various plugins).