Set a constant namespace for module sourcemaps

[pierlon]

Description

At the moment, sourcemap namespaces are generated with the value from output.library, which becomes: webpack://wp.[name]. Firefox is unable to retrieve the sourcemaps because it is an invalid URL, for example:

Source map error: TypeError: Invalid URL: webpack://wp.[name]/webpack/bootstrap
Resource URL: http://localhost:8888/wp-content/plugins/gutenberg/build/components/index.js?ver=1565639281
Source Map URL: index.js.map

This PR sets the namespace to wp, so the URL becomes webpack://wp

N.B.: Chrome is not affected.

[pablinos]

I'm not sure if there are other implications with this change, but I was having the same issue with Firefox, and I can confirm that this fixes it. The source maps are also still loaded by Chrome as expected.

[gziolo]

I can confirm it works with Chrome and Firefox as advertised. It still doesn't work with Safari though.

[gziolo]

@pierlon, thank you for your first code contribution, this improvement is much appreciated 🙇