Convert to a package instead of a single file

[itamarst]

Not strictly necessary, but if #211 or follow up issues ends up being sufficiently complex, would be nice if it wasn't just one big module.

[ogrisel]

The motivation to make it a single module was to make it easier to vendor. I think we should check if they are any vendored copies of threadpoolctl before making a decision to switch to a more complex package structure.

[itamarst]

Makes sense. I'll proceed without waiting for this.

I do think it's worth considering; https://pypi.org/project/vendoring/ is decent these days (from my vague memory of using it for pip's internal vendoring.)