def dep_to_source(dep, source_dir, dep_dir)
      stub = dep.gsub(dep_dir, source_dir).ext('')
      src = stub_to_src(stub)
      return src if src
      raise "No matching source for #{dep} found."
    end
def dep_to_source(dep, source_dir, dep_dir)
      stub = dep.gsub(dep_dir, source_dir).ext('')
      src = stub_to_src(stub)
      return src if src
      raise "No matching source for #{dep} found."
    end
Transforms a dependency file name into its corresponding source file name by replacing
 file name extension and object directory with dependency directory.
 Searches through list of source files to find it.

 @param [String] dep         Source filename
 @param [String] source_dir  Project source base directory
 @param [String] dep_dir     Project dependency base directory
 @return [String]            Mapped filename