Inconsistent default behavior of include

[radoering]

Before diving into the issue, a short clarification:

1. There are packages includes, which default to sdist and wheel.
2. There are includes, 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 (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:

• Add support of inline tables in include on pyproject.toml. poetry-core#6 added support for includes and describes that they should be only sdist per default. This PR has already contained the bug.
• Includes default format 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.
• Add documentation on inline tables in include. Resolves: Core #8. #2268 added docs that it defaults to sdist and wheel.
• Fix documentation for default format option for include and exclude #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