Support for nested components in angle bracket invocation syntax

[tomdale]

This is an implementation of the Nested Invocations in Angle Bracket Syntax RFC. It enables invoking components nested inside directories with angle bracket invocation syntax.

For example, if a component exists in app/templates/components/foo-bar/baz-bang, you can invoke it with curlies like this:

{{foo-bar/baz-bang}}

This PR adds support for invoking that same component with angle brackets like this:

<FooBar::BazBang />

The feature is added behind the EMBER_GLIMMER_ANGLE_BRACKET_NESTED_LOOKUP feature flag.

[rwjblue]

Has conflicts with the PR just merged (which removes some of the other previously enabled features).

[tomdale]

@rwjblue Sorry, will fix.

[rwjblue]

Also, I'd strongly prefer coming to a conclusion in the RFC before implementing (unless we are also accepting a feature enabling the same feature with /).