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/dff_buf_array.py

@@ -6,13 +6,13 @@
 # All rights reserved.
 #
 import debug
-import design
-from vector import vector
+from base import design
+from base import vector
 from globals import OPTS
 from sram_factory import factory
 
 
-class dff_buf_array(design.design):
+class dff_buf_array(design):
     """
     This is a simple row (or multiple rows) of flops.
     Unlike the data flops, these are never spaced out.