support/scripts/scancpan: generate hash file

retrieve MD5 and SHA256 from metacpan.org, and store them in the hash
file for each package.

[Thomas: remove the odd indentation of the filename for the md5 hash
lines in the hash file.]

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>