Single Source File Assignment Graph and Call Target Resolution

[abhisek]

To be able to support #94 we first need to achieve:

1. Call Graph building across source files
2. Call Graph path query
3. Vulnerable function symbols in vulnerability database

This paves the path for [1]. We need to extend vet's code analysis framework to support call target resolution within a single source file. For example:

import x from y

v1 = v()
v2 = v1
v2()

Our analysis engine should be able to resolve v2 as x. Actual approach can be through assignment graph, PDG or other methodology.