Data: 3.11 emits a warning during build

[ef4]

Reproduction:

1. Using ember-cli 3.10.1 (which is the latest stable as of this writing), run ember new example-app.
2. cd example-app
3. ember install ember-data (I got 3.11.0).
4. ember build

You will see this warning printed in the terminal:

'ember-inflector' is imported by -private/build-url-mixin.js, but could not be resolved – treating it as an external dependency

which I think comes from Ember Data's rollup.

Probably ember-inflector is supposed to be an external dependency, but it needs to be configured explicitly so rollup doesn't warn.

[efx]

I see this on master as well as of 2c2cfb99ba16232fbc779371acc2e14415cfe484.

[runspired]

hrm, I swear I zapped all of these prior to releasing 3.11. Will take a poke at it later.

[jameshahn2]

Getting the same warning after upgrading to 3.11.

[rwjblue]

Will be fixed by #6242