The pip-specific Path implementation has been removed, and all its
usages replaced by pathlib.Path. The tmpdir and tmpdir_factory fixtures
are also removed, and all usages are replaced by tmp_path and
tmp_path_factory, which use pathlib.Path.
The pip() function now also accepts pathlib.Path so we don't need to put
str() everywhere. Path arguments are coerced with os.fspath() into str.
When there is a pyproject.toml, metadata preparation must be
done in the isolated build environment for legacy editable installs too
(fixes a regression).
Also detect earlier if an editable install must go through the
legacy install path, to be sure to run it in an environment
with the correct build requirements.
check_binary_allowed is only used to check
if a wheel needs to be built in 'pip install' mode.
It mixed format control and pep517 mode check.
We change it so it checks allowed formats only,
which leads to better readability of _should_build().
_should_cache is only called by _get_cache_dir.
In pip install mode, _get_cache_dir is never called when
check_binary_allowed returns False because in that case
should_build_for_install_command has returned False before
and the build was skipped.
In pip wheel mode, check_binary_allowed always returns True
(because it is not passed to the build function).
So _should_cache can use _always_true for check_binary_allowed.
*Alternative*
Alternatively, we could have passed check_binary_allowed
to build in pip wheel mode. The only difference is that wheels built
locally from *legacy* packages would then not be cached,
when pip wheel is used with --no-binary.
Separately test should_build_for_wheel_command
and should_buid_for_install_command.
The tests are simpler and this open the door
for reasoning about both functions independently.
