python: Update gem5 lib downloader for new url_base field
python: Update gem5 lib downloader for new url_base field

This patch is designed to accomodate the upcoming change to the gem5
resources resources.json file:
https://gem5-review.googlesource.com/c/public/gem5-resources/+/51167.
The change extracts the base url ('http://dist.gem5.org/dist/develop')
from the urls. This patch is both compataible with the current
resources.json scheme and the upcoming schema.

Change-Id: I2a5b87cf94ba1afcb47d1f7d3ea48d0945ff21c4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51327
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5