Add swSrc to list of file dependencies for InjectManifest

[green-arrow]

R: @jeffposnick @addyosmani @gauntface

Fixes #1428

Description of what's changed/fixed.

Currently, webpack won't rebuild when in watch mode if a custom service worker swSrc is modified. This PR adds the custom service worker to the list of watched files according to this part of the webpack docs.

I looked through the tests for inject-manifest, but I'm not entirely sure how to write a proper test that validates the functionality. I'm happy to add a test if someone can point me in the right direction!

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[green-arrow]

@googlebot - I signed it

[googlebot]

CLAs look good, thanks!

[jeffposnick]

Thanks, @green-arrow! This looks like the right approach.

(I'll kick the AppVeyor CI test to see why it's failing, but it doesn't look like it's related to your PR.)

[green-arrow]

Glad to see this make it in, thanks @jeffposnick!