Add support for index imports

Author: xzyfer

src/file.cpp

@@ -324,6 +324,8 @@ namespace Sass {
     // (2) underscore + given
     // (3) underscore + given + extension
     // (4) given + extension
+    // (5) given + _index.scss
+    // (6) given + _index.sass
     std::vector<Include> resolve_includes(const std::string& root, const std::string& file, const std::vector<std::string>& exts)
     {
       std::string filename = join_paths(root, file);
@@ -351,6 +353,25 @@ namespace Sass {
         abs_path = join_paths(root, rel_path);
         if (file_exists(abs_path)) includes.push_back({{ rel_path, root }, abs_path });
       }
+      // index files
+      if (includes.size() == 0) {
+        // ignore directories that look like @import'able filename
+        for(auto ext : exts) {
+          if (ends_with(name, ext)) return includes;
+        }
+        // next test underscore index exts
+        for(auto ext : exts) {
+          rel_path = join_paths(base, join_paths(name, "_index" + ext));
+          abs_path = join_paths(root, rel_path);
+          if (file_exists(abs_path)) includes.push_back({{ rel_path, root }, abs_path });
+        }
+        // next test plain index exts
+        for(auto ext : exts) {
+          rel_path = join_paths(base, join_paths(name, "index" + ext));
+          abs_path = join_paths(root, rel_path);
+          if (file_exists(abs_path)) includes.push_back({{ rel_path, root }, abs_path });
+        }
+      }
       // nothing found
       return includes;
     }