getdeps: add an install_fb_python_executable() function to the CMake utilities
getdeps: add an install_fb_python_executable() function to the CMake utilities

Summary:
Add a `install_fb_python_executable()` function to `FBPythonBinary.cmake` for
helping to install python executables generated with
`add_fb_python_executable()`.  This primarily helps by automatically looking
up the correct output file to install from the generated targets.

Reviewed By: wez

Differential Revision: D18774539

fbshipit-source-id: 4b397580d72ac448f21d1db6d2cdd653cf3635df

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly