Python packages no longer use SAGE_DESTDIR except for staging/removing the .whl file.

This last bit of staging leads to sage -pip freeze showing wheel URLs in temporary build locations:

$ ./sage -pip freeze
alabaster @ file:///Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/local/var/tmp/sage/build/alabaster-0.7.12/src
appdirs==1.4.4
appnope @ file:///Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/local/var/tmp/sage/build/appnope-0.1.0.p0/src
argon2-cffi==20.1.0
attrs @ file:///Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/local/var/tmp/sage/build/attrs-19.3.0/src
Babel @ file:///Users/mkoeppe/s/sage/sage-rebasing/worktree-algebraic-2018-spring/local/var/tmp/sage/build/babel-2.6.0/src

(These locations are recorded in direct_url.json according to https://www.python.org/dev/peps/pep-0610/; see https://packaging.python.org/en/latest/specifications/direct-url/)

In this ticket, we remove the staging, writing the .whl file directly into SAGE_VENV/var/lib/sage/wheels. (Uninstallation of the .whl file will be done by adding to the spkg-piprm script.)

Thus, sage -pip freeze will show the actual file URLs of the stored wheels.

CC: @jhpalmieri @tobiasdiez

Component: build

Issue created by migration from https://trac.sagemath.org/ticket/30956