This patch replaces all instances of `from scipy.fftpack import ...`
by `from scipy import fftpack` followed by the use of qualified
names (except in the `scipy.fftpack` submodule itself).  The idea is
to simplify the replacement of `scipy.fftpack` by the FFTW-backed
`pyfftw.interfaces.scipy_fftpack`, which provides the same interface.

Currently, acceleration of e.g. `scipy.signal.fftconvolve`
requires fixing individual functions in `scipy.signal.signaltools`
(`scipy.signal.signaltools.fftn = pyfftw.interfaces.scipy_fftpack.fftn;
scipy.signal.signaltools.ifftn = pyfftw.interfaces.scipy_fftpack.ifftn`)
because `signaltools` imports the functions out of `fftpack`.
With the patch, a simple `scipy.fftpack.__dict__.update({name:
getattr(pyfftw.interfaces.scipy_fftpack, name) for name in
pyfftw.interfaces.scipy_fftpack.__all__})` (which could be wrapped in a
helper in pyfftw's side) should suffice.