Add a way to programattically cache and query for arch files

[olofk]

VTR (and by extension symbiflow) requires the user to specify a path to an xml file. This is problematic both because users need to handle these files themselves and because symbiflow might be executed on a different machine than the user uses. Both these cases are blockers for adding a VTR backend to Edalize. I did a quick hack to programmatically cache and query the arch description files here. Feel free to use it in whole or in parts. Not sure where sucha thing would best be stored but I guess here, since they are related to the arch defs files