Handle dynamic import or importSync of missing module

[thoov]

If a module is imported via import('foo') or importSync('foo') but foo cannot be resolved, instead create a dummy module that throws a runtime error stating that the module could not be found. The reasoning behind this change is that it is acceptable to have "guarded imports". An example is take ember-exam which does not explicitly depend on either ember-qunit or ember-mocha but has code like the following:

let emberTestFramework;

if (macroCondition(dependencySatisfies('ember-qunit', '*'))){
  emberTestFramework = importSync('ember-qunit');
} else if (macroCondition(dependencySatisfies('ember-mocha', '*'))) {
  emberTestFramework = importSync('ember-mocha');
}

During dev builds we do not strip out the conditionals that are falsy and thus when macros-babel-plugin runs it will try and resolve these imports (and cause a build-time error). This differs from 2 points of perspective: 1) classical system allow requiring a module that does not exist and you will not see any errors until that require is encountered at runtime (or if it is guarded it may never error). 2) The behavior of the macro system in production mode would hide the fact that this module was not found since it striped out the import branch which is different from how it would behave under dev mode.

Additional background on ember-exam's change: ember-cli/ember-exam#599

Fixes: #543
Fixes: #544

[ef4]

Overall looking good, if you can incorporate these two suggestions and get the tests green.

[thoov]

@ef4 this should be ready to merge