Relax module naming restrictions

Forcing module names to be valid ninja names is an unnecessary
restraint on the project build logic.  Allow any string as a
module name, and sanitize and uniquify the module name for use
in module-scoped variables.

Also move the module scope to be per-module instead of per-group
so that modules can use the same local variable name for each variant.

Change-Id: If44cca20712305e2c0b6d6b39daa5eace335c148