Add plugin paths to the beginning of the code path

The goal is to make sure the right application is picked, in case the
same one is available as both a RabbitMQ plugin and an external Erlang
application. For instance, this could be the case with the Cowboy
application: a version is available in the plugins, but the user could
add an incompatible version to Erlang/OTP libdir or set ERL_LIBS to
point to it.

There's one exception currently: eldap. This application used to be
available as a 3rd party one. But since Erlang R15B01, it's included
in the standard library. We trust this version to have a stable API.
Therefore, if the node runs on R15B01 or later and eldap version is
1.0+, we use this one. In all other cases, we don't trust it and prefer
the RabbitMQ plugin.
Only check ERTS version to determine if we accept an external eldap
Remove "Plugins: adding to code path" log message
No need to load/unload the plugin around the plugin_provided_by_otp/1 check

While here, remove the unused PluginVsn variable.
Merge bug26265