revert: abstract Project class and `_providers_base()`

[mr-cal]

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This is a partial revert of 'aeae554 feat!: validate devel bases (#302)'.

The base validation from #302 has not been reverted, but there is now a default implementation for Project._providers_base() such that applications do not need to subclass the Project model.

The default naming convention for bases is name@channel. Examples:

• ubuntu@24.04
• ubuntu@devel
• almalinux@9

Charmcraft is the only public application that can use this default _providers_base().

A default BuildPlanner will come in a subsequent PR.

(CRAFT-2824)

[mr-cal]

I'll wait for #321 to land first