restrict search-range to valid layers when looking up a symbol
restrict search-range to valid layers when looking up a symbol

(trac#123)



h3rb/openshadinglanguage,tectronics/openshadinglanguage,qdtk/openshadinglanguage,h3rb/openshadinglanguage,tectronics/openshadinglanguage,h3rb/openshadinglanguage,qdtk/openshadinglanguage,code-google-com/openshadinglanguage,code-google-com/openshadinglanguage,qdtk/openshadinglanguage,code-google-com/openshadinglanguage,h3rb/openshadinglanguage,tectronics/openshadinglanguage,qdtk/openshadinglanguage,code-google-com/openshadinglanguage,tectronics/openshadinglanguage