Make subdirectory path optional when using @namespace/template syntax

Problem/Motivation

If a developer organizes their component system's templates with sub-directories, it is required to use the sub-directory path when referencing the template.

For example, if a theme configure the @components namespace to point at THEME/components and places a box.twig template in THEME/components/containers/box/box.twig, then a developer would be required to use the following syntax to include that template:

{{ include("@components/containers/box/box.twig", ...) }}

However, component system designers never re-use component names, so there is not going to be two different templates named the same; e.g. only one box.html.twig file will exist in the system.

So while, organizing templates in sub-directories makes sense, having to use the sub-directory path every time you want to use that component template is annoying and not necessary to differentiate between components.

Proposed resolution

Make it possible for developers to only have to use the namespace and the template name when referencing a component's template.

{# Syntax required with Components 8.x-2.x: #}
{{ include("@components/containers/box/box.twig", ...) }}

{# Proposed syntax with Components 3.x #}
{{ include("@components/box.twig", ...) }}

Remaining tasks

On-the-fly searching through multiple directories to find the correct template is going to be a performance hit, so we need to introduce a cache that stores all the paths to template files.

API changes

Both the old syntax (with directory path) and the new syntax would work for Twig's include, embeds and extends.