pip_package: modularize build script to allow distros to install more flexibly

Gentoo Linux handles python modules slightly differently and packaging
wheels is complicated. We prefer to run setup.py directly ourselves
rather than build a wheel and then install from there.

This modularizes build_pip_package.sh to allow running parts separately.
using --src srcdir will prepare the package in a known dir so the distro
package can take it from there. If only dstdir is given (either with
--dst or as the only argument to preserve backwards compat) then
behaviour is the same as before, the sources are prepared and the wheel
is built and placed in dstdir.

Signed-off-by: Jason Zaman <jason@perfinion.com>