python: make every view a `venv` (#44382)

#40773 introduced python-venv, which improved build isolation and avoids issues with,
e.g., `ubuntu`'s system python modifying `sysconfig` to include a (very unwanted)
`local` directory within the default install layout.

This addresses a few cases where #40773 removed functionality, without harming the
default cases where we use `python-venv`.

Traditionally, *every* view with `python` in it was essentially a virtual environment,
because we would copy the `python` interpreter and `os.py` into every view when linking.
We now rely on `python-venv` to do that, but only when it's used (i.e. new builds) and
only for packages that have an `extends("python")` directive.

This again makes every view with `python` in it a virtual environment, but only
if we're not already using a package like `python-venv`. This uses a different
mechanism from before -- instead of using the `virtualenv` trick of copying `python`
into the prefix, we instead create a `pyvenv.cfg` like `venv` (the more modern way
to do it).

This fixes two things:
1. If you already had an environment before Spack `v0.22` that worked, it would
   stop working without a reconcretize and rebuild in `v0.22`, because we no longer
   copy the python interpreter on link. Adding `pyvenv.cfg` fixes this in a more
   modern way, so old views will keep working.

2. If you have an env that only includes python packages that use `depends_on("python")`
   instead of `extends("python")`, those packages will now be importable as before,
   though they won't have the same level of build isolation you'd get with `extends`
   and `python-venv`.

* views: avoid making client code deal with link functions

Users of views and ViewDescriptors shouldn't have to deal with link functions -- they
should just say what type of linking they want.

- [x] views take a link_type, not a link function
- [x] views work out the link function from the link type
- [x] view descriptors and commands now just tell the view what they want.

* python: simplify logic for avoiding pyvenv.cfg in copy views

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

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