We're down to just declaring the data files in python now.

setup.cfg can, uniquely, retrieve version info by trying to parse the
AST for simple assignments (which we use) instead of importing the
entire module.

Since the testsuite no longer tries to import a *script* as a *module*,
there is no need to check if we are in __main__ before running setup().

And don't run a pointless test to verify that the hardcoded list has
been manually maintained correctly. The same test rules used there can
translate directly to find_packages pattern rules.

Currently mesonlib does some import tricks to figure out whether it
needs to use windows or posix specific functions. This is a little
hacky, but works fine. However, the way the typing stubs are implemented
for the msvcrt and fnctl modules will cause mypy to fail on the other
platform, since the functions are not implemented.

To aleviate this (and for slightly cleaner design), I've split mesonlib
into a pacakge with three modules. A universal module contains all of
the platform agnositc code, a win32 module contains window specific
code, a posix module contains the posix specific code, and a platform
module contains no-op implementations. Then the package's __init__ file
imports all of the universal functions and all of the functions from the
approriate platform module, or the no-op versions as fallbacks. This
makes mypy happy, and avoids `if`ing all over the code to switch between
the platform specific code.

Do this by tracking CMAKE_CURRENT_{SOURCE,BINARY}_DIR variables.
This is achieved by injecting CMake code with CMAKE_PROJECT_INCLUDE
and overriding some builtin functions with a wrapper that adds
additional trace information.