Revert "changed help_provider to use rospkg if available"

This reverts commit c686e6a2c80bb37c03843cc8dbdf1fc9595e9ac9.
Revert "fixed help_provider for C++ plugins by reducing the difference of the plugin_descriptor attributes between Python and C++ plugins"

This reverts commit 295a040845dd2290de9c22e80bd41d7c53f413db.
The previous commits were reverted until the problems they cause are fixed
0.2.15