Build/Test Tools: fix source map loading in Firefox.

Fix an issue that prevented source maps from loading correctly in Firefox developer tools. Matches a fix introduced in Gutenberg build in PR:17024.

Props pierlo, andraganescu.
Fixes #47870.



git-svn-id: https://develop.svn.wordpress.org/trunk@46158 602fd350-edb4-49c9-b593-d223f7449a82

tools/webpack/packages.js

@@ -207,6 +207,7 @@ module.exports = function( env = { environment: 'production', watch: false, buil
 			return memo;
 		}, {} ),
 		output: {
+			devtoolNamespace: 'wp',
 			filename: `[basename]${ suffix }.js`,
 			path: join( baseDir, `${ buildTarget }/js/dist` ),
 			library: {