Fix fallback to old inspectors

[wycats]

Closes #1120

As @chancancode explained in #1120, the injected debug code previously did
too much work in order to report a version miss.

To help understand what went wrong, I'll give an abridged version of the
boot-up protocol, and how the fallback logic fits in.

1. The Ember inspector application boots up inside of a devtools iframe,
   and injects the version of ember_debug.js for the latest version
   of the Ember inspector.
2. ember_debug.js to become aware of any supported version of Ember,
   and when that happens, it checks whether the version of Ember on the
   current page is supported by the current version of ember_debug.js.
3. If it matches, everything proceeds as usual, and ember_debug.js
   starts communicating with the inspector pane.
4. If it doesn't match, ember_debug.js sends a version-mismatch message to
   the inspector pane along with the current version of Ember used on the current
   page. The inspector pane then calculates which version of the inspector to use
   (currently either 0-0-0 for Ember 0.0 to Ember 2.6 or 2-7-0 for Ember 2.8
   to Ember 3.2). When this happens, the inspector navigates itself to the older
   version of the inspector, allowing that version to bootstrap itself.

Note that the inspector uses the Ember registry and loader to register its
modules, so any modules for the latest version of ember_debug will still be
registered when the older version of ember_debug.js is evaluated.

This is not intrinsically a problem, since the calls to define in the new
version of ember_debug.js will replace the old modules. Any modules that were
present in the latest version but not the previous version will still be
registered, but won't be used by the older code.

The problem with all of that is that if any module is required in the process
of reporting the version miss, the module's exports will remain cached in
Ember's loader. Replacing the module with new source code has no effect,
because the next time someone tries to require the module, it will just return
the old cached exports, and the new define is completely ignored.

This means that, depending on which code is required as part of reporting the
version miss, a random Frankenstein combination of modules will be used at
runtime. It should come as no surprise that this could have problems. The
reason that it didn't cause any issues until recently is a coincidence: the
assortment of modules in question happened to work well enough to avoid any
problems that incited a user to report the bug.

The solution to the problem in this commit is to completely avoid requiring any
modules before the version is correctly matched. There were three modules
previously required:

1. Two utility modules: on-ready and version. These modules were small so I
   inlined the code next to the version reporting logic.
2. The ember_debug adapter. This is a much thornier dependency, because it sucks
   in a whole bunch of additional dependencies and is the root cause of the current
   issue. I addressed this dependency by hand-coding just enough logic to create
   a MessageChannel, post it to the current window, and postMessage a
   version-mismatch message through the channel.

This works as long as the adapter uses a MessageChannel to communicate the
version mismatch. This is how web-extension.js works, which covers Chrome,
Firefox, Edgium, and remote debugging using the remote debugging protocol.

It does not cover other backends, but those backends are currently broken for
other reasons. It doesn't substantially burden the work of making other
backends work again, it simply constrains backends to use a MessageChannel
(posted to the inspected app's window) to communicate version mismatches. It
doesn't otherwise constrain any adapter functionality in ember_debug or the
Ember Inspector. If this constraint isn't acceptable for a future backend, we
could also support alternative ways of communicating the version mismatch, as
long as that didn't require the version reporting code to load in the entire
adapter module.

[wycats]

I tested this by creating a new Ember 3.3 app, copying the 2-7-0 contents into the dist directory. I then reproduced the problem with the requireModule lines in place. I then removed the requireModule lines and verified that the problem no longer manifested.

[RobbieTheWagner]

Looks good. Thanks for tackling this! Just one question about importing and using some existing code.