Allow arbitrary compositions in TemplateLoader template names.

• Allows compositing any two templates in TemplateLoader, and not just filters: 'A|B|C' is now valid for any template A, B and C.
• Allows parents to be composited paths 'A|B|C'. In such a schema, if A or B themselves specify a parent, the actual composition becomes A|pA|B|pB|C and so on.

This allows, by leveraging the common schema of our entities, to reduce duplication.

For convenience, templates in "special/filters/" and "mixins/" can be included by their direct name. Others have to be completely specified.

See the two provided cases for examples:

• 'hoplite' becomes a mixin that can be used to apply the Phalanx formation
• 'builder' becomes a mixin that can be give a template the ability to build the standard structures, and gives the 'Builder' identity class. This also allows deduplicating that list of tokens.

Update checkrefs & swap std::map for std::unordered_map in TemplateLoader.

Differential Revision: https://code.wildfiregames.com/D3801