Allow parsing Blueprints files without evaluating

Running bpfmt or bpmodify on a Blueprints file that references
variables defined in a parent Blueprints file causes parse errors
on unknown variables.  Modify the parser to parse in two modes,
parser.Parse and parser.ParseAndEval.  The first parses without
attempting to evaluate variables and expressions, the second
performs a full evaluation.

Change-Id: Ic11948ea77c8e65379d7260591ada15db0e4f5b9