Ensure dist contents are not included in govuk directory

This commit fixes the issues identified while trying to run
`npm run build:dist`.

Current and desired directory structure:

dist
├── assets
│   ├── fonts
│   └── images
├── VERSION.txt
├── govuk-frontend-2.13.0.min.css
├── govuk-frontend-2.13.0.min.js
└── govuk-frontend-ie8-2.13.0.min.css

See proposal for more detail
https://github.com/alphagov/govuk-design-system-architecture/blob/master/proposals/005-namespace-govuk-frontend-nunjucks-sass-using-a-nested-folder-structure.md

tasks/gulp/__tests__/after-build-dist.test.js

@@ -18,7 +18,7 @@ describe('dist/', () => {
           files => {
             return files
               // Remove /package prefix from filenames
-              .map(file => file.replace(/^src\/assets\//, ''))
+              .map(file => file.replace(/^src\/govuk\/assets\//, ''))
               // Sort to make comparison easier
               .sort()
           },