ext: simplify mapping/template discovery

[slint]

The recursive functions that walk/parse templates/mappings are a usual source of confusion and are hard to tweak when changes or new features are introduced (e.g. supporting multiple ES versions, index/alias prefixing, etc).

In the end how most people would think about this problem would be something like:

1. Build a tree with all mappings/templates (apply ES versioning logic here)
2. Create the mappings/templates from the tree leaves (apply prefixing here)
3. Create aliases based on tree roots and their leaves (apply prefixing here)

Of course any refactoring should include tests to prevent breaking existing behaviour.