search: refactor PostQueryBuilder class methods into instance methods.

* Make scan_query, parse_query, normalize_query into instance methods
  instead of class methods. This is to a) clean up the API and b)
  prepare for moving certain tag utility methods into PostQueryBuilder.

* Fix a few cases where a caller used scan_query when they should have
  used split_query or parse_tag_edit.

app/logical/post_sets/post.rb

@@ -4,7 +4,7 @@ module PostSets
     attr_reader :tag_array, :page, :raw, :random, :post_count, :format
 
     def initialize(tags, page = 1, per_page = nil, raw: false, random: false, format: "html")
-      @tag_array = PostQueryBuilder.split_query(tags)
+      @tag_array = PostQueryBuilder.new(tags).split_query
       @page = page
       @per_page = per_page
       @raw = raw.to_s.truthy?