Fix search results in root dir & deprecate OC_Search_Provider

On master clicking on a search result located in the root folder will show you a blank page because the query parameter is `dir=` instead of `dir=/`. Subdirectories work fine. 
0e2f51b fixes this cornercase
b56f5f2 just adds a deprecation warning to the legacy OC_Search_Provider

@PVince81 @schiesbn quick review of 7 LOC?