Fix dependency installation during setup.py install

The pwb_install function introduced in Idd1794481b4b57c001fed8df0bf8178bb5eccc30
did not take into account the introspection that happens in install.install.run:
depending on the caller, it changes behavior - in this case to the *incorrect*
behavior. We now explicitly call the right backend function.

See also: https://github.com/jaraco/setuptools/blob/master/setuptools/command/install.py#L50

Bug: 56386
Change-Id: Ib27fd645c51f0fd7a903f7118c04f64f96501f89
Merge "Fix dependency installation during setup.py install"