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/tri_gate_array.py

@@ -7,12 +7,12 @@
 #
 import debug
 from tech import drc
-import design
-from vector import vector
+from base import design
+from base import vector
 from sram_factory import factory
 from globals import OPTS
 
-class tri_gate_array(design.design):
+class tri_gate_array(design):
     """
     Dynamically generated tri gate array of all bitlines.  words_per_row
     """