Ensure `variantlib` is vendorable

[rgommers]

When working on the code in this repo, let's keep in mind that variantlib needs to be both reimplementable (e.g., in Rust) and vendorable (e.g., if a build frontend or backend cannot have dependencies, like pip).

I didn't audit all the code so this is just a single example - an absolute import like this won't work well, it breaks when variantlib is vendored:

from variantlib.loader import PluginLoader

[mgorny]

I think pip uses a script to adjust imports when vendoring.

[DEKHTIARJonathan]

I checked a few other vendored dependencies of pip and it does seem indeed that most use relative imports from . import X, Y, Z

I do personally prefer absolute imports but ultimately whatever the pip maintainers want

It's not necessarily a big issue to fix - but it's a big idea to keep an eye on this. Thanks for opening the issue @rgommers

[rgommers]

Note that it's not just pip. For example, PDM and scikit-build-core both vendor pyproject-metadata; I'd expect them to also want to vendor variantlib.

It's not necessarily a big issue to fix - but it's a big idea to keep an eye on this.

Agreed, not hard to fix. Just friendly to keep in mind as we go

[DEKHTIARJonathan]

Makes a lot of sense. Thanks for that comment Ralf. I indeed totally did not think about that.
Good news is that it's a trivial change :D

[Secrus]

If you use a tool like vendoring (which is what pip uses), it automagically resolves the import paths, so the issue is mostly about manual vendoring.

[DEKHTIARJonathan]

@mgorny I believe that's the tool you were talking about right?

[rgommers]

so the issue is mostly about manual vendoring.

Agreed - just a plain copy, or adding it as a git submodule.