Treat default dependencies as main group

[Lykos153]

Poetry documentation on dependency groups [1] says:
"The dependencies declared in tool.poetry.dependencies are part of an
implicit main group."

The current implementation does not account for that and in particular does not allow to skip installation of default dependencies like in one of
poetry install --without main
poetry install --only-root
poetry install --only othergroup.

This commit adds main as an implicit group and also adds it as a default value where applicable.

[1]
https://python-poetry.org/docs/managing-dependencies#dependency-groups

Closes #1578

Contribution checklist (recommended but not always applicable/required):

• There's an automated test for this change
• Commit messages or code include references to related issues or PRs (including third parties)
• Commit messages are conventional - examples from the log include "feat: add changelog files to fixup hook", "fix(contourpy): allow wheel usage", and "test: add sqlalchemy2 test"

[Lykos153]

A more backwards compatible solution would be to add a flag like useDefaultDeps ? true. Though in this case the behavior is not completely obvious, because poetry also installs by default all groups that don't have optional = true.

[Lykos153]

Marking as ready for review in order to get feedback on whether the approach in this PR would be accepted or if a flag as proposed in my last comment would be preferred.

[cpcloud]

The initial approach here seems reasonable to me.

[cpcloud]

Thanks!