pkg-infra: move the hg download helper to a script

Maintaining the download helpers in the Makefile has proved to be a bit
complex, so move it to a shell script.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
[tested a particular scenario that used to fail, when the 'hg archive'
step is interrupted, now working fine]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>