IDL compiler: Move include path handling to compute_dependencies

This is a preliminary CL for partial interfaces in the Python IDL compiler.

Currently there is code duplication:
compute_dependencies.py, idl_compiler, and code_generator_v8.py
...all do path handling.

This consolidates it into compute_dependencies.py, so now the compiler
doesn't need to know anything about paths.

Specifically, this CL means that when we compute and store the full path,
we also compute and store the C++ include path for the header.


This refactoring will make adding partial interfaces to Python easy:
we need include paths of partial interfaces (as this in included in
the generated code), and we can store them in the same way.
(Currently full path to dependencies is stored.)

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/137993002

git-svn-id: svn://svn.chromium.org/blink/trunk@165064 bbb929c8-8fbe-4397-9dbb-9b2b20218538