When behind a reverse proxy that terminates SSL, the server sees HTTP
requests internally. Added _get_base_url() helper that respects the
X-Forwarded-Proto header to generate correct external HTTPS URLs.
This fixes links in the PyPI simple index showing http:// instead of
https:// when accessed via HTTPS through a load balancer.
Artifactory and other registries may return relative URLs in their
Simple API responses (e.g., ../../packages/...). The proxy now resolves
these to absolute URLs using urljoin() before encoding them in the
upstream parameter.
This fixes package downloads failing when the upstream registry uses
relative URLs in its package index.
