flit_core: respect PIP_INSTALL_DIST_PATH

flit_core: respect PIP_INSTALL_DIST_PATH

pip_install_wheel expects the wheels to be in PIP_INSTALL_DIST_PATH but
this class was writing to the same directory through chance not design.

Respect PIP_INSTALL_DIST_PATH as the output directory, and use
[cleandirs] to ensure that it exists and is empty.

Also call the flit_core.wheel module directly as this is a public entry
point to the wheel building logic.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>