Source map support for injectManifest makes incorrect assumptions

**Library Affected**:
workbox-build

**Browser & Platform**:
n/a

**Issue or Feature Request Description**:
It looks like source map support was added for the Webpack plugin in #2239 when injecting the manifest. However, there is no such support for the `injectManifest` method in the _workbox-build_ package. It would be helpful if the same code could be applied there, i.e. if a source map exists for `swSrc` then write an adjusted one for `swDest`.