WIP: new variant resolving / rendering

[wolfv]

This is an attempt to separate things better into "stage 1" and "stage 2" rendering.

Stage 1:

Get all used variables from Jinja and conditional expressions and then render these out for all combinations that are found based on the variant config. This already creates a number of different combinations.

Stage 2:

From the rendered recipes (no more Jinja, conditionals) we need to get all the requirements at build/link time and create additional variants.

Stage 3:

Topologically sort and de-duplicate outputs (with multiple outputs, these renders might create duplicate outputs, for example if you have a C library and multiple variants for Python bindings).

Note to self:

We need to make sure that we keep this logic from main:

                // We also replace `-` with `_` in the keys for better compatibility
                // with conda-build and because then we can set them directly as env vars
                // and the same for `.` because we want to be able to set them as env vars directly
                let used_filtered = used_filtered
                    .into_iter()
                    .map(|(k, v)| (k.replace("-", "_"), v))
                    .map(|(k, v)| (k.replace(".", "_"), v))
                    .collect::<BTreeMap<_, _>>();

[wolfv]

Note: missing in this PR are updated handling of - and . in used variables :)