bitbake/fetch2: Ensure that mirror fetches are symlinked from the download directory
When files are fetched from a mirror source that happens to be local,
ensure links are created for the file since subsequent fetch calls
can then follow the links to find files.
Any other approach such as the existing manipulations of localpath
internally to the fetcher are prone to errors, races and other issues.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
