Inconsistent default behavior of `include`

Before diving into the issue, a short clarification:



1. There are [packages includes](https://python-poetry.org/docs/pyproject/#packages), which default to sdist **and** wheel.

2. There are [includes](https://python-poetry.org/docs/pyproject/#include-and-exclude), which (according to the docs) default to **only** sdist.



These different defaults can be confusing, but might make sense because you normally want to include packages in sdists and wheels but additional data like tests and a changelog only in the sdist.



# Issue



The behavior for [includes](https://python-poetry.org/docs/pyproject/#include-and-exclude) (the second one) is different for files and directories:

- files are included in both sdist and wheel

- directories are only included in sdists



Considering that the docs were "fixed" recently in #8852 to describe the behavior for directories - previously, it described the behavior for files - I wondered which behavior is correct and which is the bug?



Events of confusion in chronologically order:



- python-poetry/poetry-core#6 added support for includes and describes that they should be only sdist per default. This PR has already contained the bug.

- python-poetry/poetry-core#114 changed the default behavior for packages includes (first one), but confusingly added a test for includes (second one) - actually testing the bug instead of the change. Thus, we cannot trust the test.

- #2268 added docs that it defaults to sdist and wheel.

- #8852 changed the docs that it defaults to only sdist.



We can be sure that the initial intention was to default to "only sdist". However, I am not sure if the behavior should be changed to "sdist and wheel" later or if it was just a confusion between the two types of includes.



Thus, the big question: **What should the default be?**



- sdist and wheel for package includes (first type) and sdist only for includes (second type)

  - pro: You normally want to include packages in sdists and wheels but additional data like tests and a changelog only in the sdist

  - contra: different defaults

- sdist and wheel for both types of includes

  - pro: same defaults

  - contra: maybe improper default for one type

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