small cleanup
BaseLoader.load_template always returns source instead of compiled template now
working state
removed dependencies on template_source_loaders
removed template_source_loaders global variable
working state
working state
finally moved make_origin and find_template_loader functions from loader to base module

These functions were copied into base module in previous commit,
but we could not remove old ones since loader.cached was importing these
functions from loader module.
small clean up: moved documentation
removed builtins global variable
removed dependence on get_template in inclusion tags

Test RequestContextTests doesn't use default_engine at all.
rewritten TemplateEngine.find_template so it always returns compiled template instead of raw template source
removed TemplateEngine.find_compiled_template since find_template method does the same job
loader.get_template delegates to default_engine.find_template now
_calculate_template_source_loaders become a function independent from TemplateEngine class
rewritten TemplateTagLoading so it doesn't rely on default_engine
loader.select_template function delegates to new TemplateEngine.select_template method now
rewritten test_include_missing_template test so it doesn't rely on default_engine
rewritten test_loader_debug_origin so it doesn't rely on default_engine

(But the cached loader relies on default_engine.)
rewritten test_extends_include_missing_baseloader so it doesn't rely on default_engine