When cross-compiling, the local Python interpreter that is used
to run `ensurepip` may not have the same value of `sys.prefix` as
the value of the 'prefix' variable that is set in the Makefile.

With the following values used to install Python locally for
a later copy to the files hierarchy owned by the 'termux'
application on an Android device:

    DESTDIR=/tmp/android
    prefix=/data/data/com.termux/files/usr/local

'make install' causes ensurepip to install pip in
$(DESTDIR)/usr/local instead of the expected $(DESTDIR)/$(prefix)
where is installed the standard library.

The attached patch fixes the problem. The patch was implemented
assuming that pip uses distutils for the installation (note that
setup.py also uses the --prefix option in the Makefile), but I
know nothing about pip so forgive me if the patch is wrong and
please just assume it is just a way to demonstrate the problem.

Fixes: python#75229
Fixes: https://bugs.python.org/issue31046
Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
Co-authored-by: Zackery Spytz <zspytz@gmail.com>
References: python#17634
Signed-off-by: Matěj Cepl <mcepl@cepl.eu>

When using `python -m ensurepip` with the `--root` option for staged
installations, the generated pip script contained an incorrect shebang
that pointed into the staging directory. This made the installation
unusable once the staging directory was removed.

This commit fixes the issue by using the internal pip `--executable`
option to force the shebang to point to the correct, final
interpreter path.

It also corrects related pathing issues:

- Removes the check that incorrectly disallowed using --root and
  --prefix together.
- Defaults the installation prefix to `/` when --root is used alone,
  ensuring installation occurs at the base of the staging directory.

References: python#17634 (comment)
Signed-off-by: Matěj Cepl <mcepl@cepl.eu>