Make UpstreamSource intermediate specs from upstream

[tedgarb]

As discovered in #336, the retirement of the v1 dependencies API revealed that UpstreamSource, when the dependencies API is unavailable, tells the client to go talk to rubygems directly for the spec files. This model does not work in general, because there is no guarantee that the client can access rubygems directly. This commit changes UpstreamSource to intermediate the connection

Description:

---

Tasks:

• Describe the problem / feature
• Write tests

I will abide by the code of conduct.

[indirect]

Have you tested this? I don't think it will allow installation of gems because the /quick/Marshal.4.8/ specs aren't also proxied.

[tedgarb]

quick/Marshal is handled in that class via the cache mechanism in

gemstash/lib/gemstash/gem_source/upstream_source.rb

Line 123 in 523a7e6

def serve_marshal(id)

This patch is identical to the code I am running in production in my environment today, which is letting me install gems and to search for them

[indirect]

Ah, got it. Thanks for explaining!