Update copy:files to glob “src” not “src/govuk”

[colinrotherham]

This PR widens gulp copy:files to glob ./src/**/* not just the "govuk" directory

Why? Since #2851 was merged we're now running build tasks on files within:

./src/govuk
./src/govuk-prototype-kit

As suggested by @36degrees in #2851 (review) we could remove some unnecessary file operations:

(I think it'd be neater if the .js and .scss files were handled by the existing copy-files task, but that looks like it's caught up in the fact that config.src is actually src/govuk so happy to revisit that later)

These changes were made possible by:

• Use absolute directory paths everywhere #2962

[colinrotherham]

@36degrees Does this PR address your code review suggestion?

[36degrees]

👍🏻