Use packages for imports.

Must set PYTHONPATH to include OPENRAM_HOME now.
Reorganizes subdirs as packages.
Rewrites unit tests to use packages.
Update README.md with instructions, dependencies etc.
Update sky130 module imports.
Change tech specific package from modules to custom.

compiler/modules/bitcell_base_array.py

@@ -6,12 +6,12 @@
 # All rights reserved.
 #
 import debug
-import design
+from base import design
 from sram_factory import factory
 from globals import OPTS
 
 
-class bitcell_base_array(design.design):
+class bitcell_base_array(design):
     """
     Abstract base class for bitcell-arrays -- bitcell, dummy, replica
     """