When loading multiple libraries built with webpack, you can run into
collisions of the sourcemap file paths. For examle, both have
"webpack:///src/index.js".
This change addresses the problem by introducing a new output option
`output.devtoolNamespace` which defaults to `output.library` when
not specified. The defaults moduleFilenameTemplates in all the
sourcemap plugins have been modified to start with:
"webpack://[namespace]/...", where [namespace] will be replaced by
the `output.devtoolNamespace`.
Notice that there are only two slashes following "webpack:" now.
This is to make it behave just as before when not building with a
namespace. When building with a namespace you only get the two
slashes, but from what I've seen the chrome dev tools only care
about the first 2 slashes anyways.
Discussed with sokra here:
https://github.com/webpack/webpack/issues/5767
Stephan Badragan