`importSync()` is intended to work like `import()` (buy sync),
which, in turns, is expected to work like a namespace import –
`import * as NS from "foo";`. In general, we would expect that to
be interchangable with `const NS = importSync("foo");`.

Prior to this commit, this is not true for commonjs packages:

```js
import * as QUnit1 from "qunit";
// => { QUnit, assert, begin, config, ... }

QUnit1.assert // => function

const QUnit2 = importSync("qunit");
// => { default: { QUnit, assert, begin, config, ... } }

QUnit2.assert // => undefined
```

Of course, using ES imports on commonjs packages is, in itself, an
ill-defined concept, but nevertheless, it is a very common interop
requirement.

The previous behavior is introduced in embroider-build#1076. The intent appears to
be fixing a different interop issue, where we would expect these to
work the same way:

```js
import QUnit1 from "qunit";
// => { Qunit, assert, begin, config, ... }

QUnit1.assert // => function

const QUnit2 = importSync("qunit").default;
// => { QUnit, assert, begin, config, ... }

QUnit2.assert // => function
```

The fix in embroider-build#1076, however, broke the exepctation that `import()`
should behave like a namespace improt statement for CJS modules.

This commit attempts to satisfy both of these requirements, with
one caveat: `importSync("foo").default` is present on the shimed
CJS modules, but in `import * as FOO from "foo";`, `FOO.default`
is undefined.

Arguably, this is a bug in webpack's implementation – if you are
able to write an import statement that references the default
export, you should be able to do the same in the namespace import
as per the spec.

So, the implementation here is different but more correct.

Fixes embroider-build#1530

In classic builds, multiple different versions of `@embroider/macros` may get smashed together, overwriting each other's files in ways that we  cannot control.

If we want packages that upgrade to get this bugfix to reliably have it, they need to use a new name to refer to the module.