Make search compatible with sphinx 2.2.1

We are relying on FILE_SUFFIX,
but we were relying on a bug actually sphinx-doc/sphinx#6741

This is compatible with older and new versions.

This is also related that we don't save the real full path when
indexing.

https://github.com/readthedocs/readthedocs.org/blob/24ca737e3f5c0df359801a8dede6519f67a70cb6/readthedocs/search/api.py#L34-L38

Fixes #6801

Author: stsewd

readthedocs/core/static-src/core/js/doc-embed/search.js

@@ -77,7 +77,13 @@ function attach_elastic_search_query(data) {
                         }
 
                         // Creating the result from elements
-                        var link = doc.link + DOCUMENTATION_OPTIONS.FILE_SUFFIX;
+                        var suffix = DOCUMENTATION_OPTIONS.FILE_SUFFIX;
+                        // Since sphinx 2.2.1 FILE_SUFFIX is .html for all builders,
+                        // and there is a new BUILDER option.
+                        if ('BUILDER' in DOCUMENTATION_OPTIONS && DOCUMENTATION_OPTIONS.BUILDER === 'readthedocsdirhtml') {
+                          var suffix = '';
+                        }
+                        var link = doc.link + suffix;
 
                         var item = $('<a>', {'href': link});