Add install, use the installed cell libs

Add install, use the installed cell libs

The extension needs to load a cell library in order to tech-map things,
and that means having a Verilog file at a locatable path. The Yosys
built-ins put them in the <share>/yosys directory, and so we install
things there and load them from there for convenience.

Using a path +/<path> will be rewritten inside Yosys to load from
<share>/yosys/<path>. I add a nangate/ sub-directory for namespacing
purposes during the install, and load things from +/nangate/<file>.