gh-84530: fix namespace package support in modulefinder (#29196)

* bpo-40350: fix namespace package support in modulefinder

Signed-off-by: Filipe Laíns <lains@riseup.net>

* consider that namespace package specs might have the loader set

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Revert "consider that namespace package specs might have the loader set"

This reverts commit 23fb4e0de38466217e932115143db1a9584e35da.

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Fix load_module and load_package to handle namespace packages

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Drop _NAMESPACE constant

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Drop importlib changes

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Update NamespacePath check

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Mixed some

Signed-off-by: Filipe Laíns <lains@riseup.net>

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Signed-off-by: Filipe Laíns <lains@riseup.net>