fix: Ensure trailing slash is added to source URIs added via gem sources

[zirni]

GitHub's private gem registry expects the first path segment after the host
to represent the namespace, typically the organization or user name. [1]

When adding a source with

  gem sources --add https://user:password@rubygems.pkg.github.com/my-org

without a trailing slash, the last path segment ("my-org") is interpreted as a
file and removed during relative path resolution. This causes the resulting
URI to become

  https://user:password@rubygems.pkg.github.com/gems/foo.gem

instead of the correct

  https://user:password@rubygems.pkg.github.com/my-org/gems/foo.gem. [2]

Example error:

  gem source -a https://user:password@rubygems.pkg.github.com/my-org
  gem install -rf foo.gem
  rubygems/remote_fetcher.rb:238:in `fetch_http': bad response Not Found 404 (https://user:REDACTED@rubygems.pkg.github.com/gems/foo-0.7.1.gem) (Gem::RemoteFetcher::FetchError)

Although this behavior complies with RFC 2396, it's incompatible with GitHub's
gem registry requirements.

The remote fetcher is just append a relative path without using ./ [3]

To address this, we automatically append a trailing slash when adding new gem
sources.

As illustrated in [4] and [5], given the base URI

  http://a/b/c/d;p?q

and a relative path

  g/f

the resolution process replaces "d;p?q" and yields

  http://a/b/c/g/f

[1] https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-rubygems-registry#authenticating-with-a-personal-access-token
[2] https://github.com/ruby/rubygems/blob/master/lib/rubygems/vendor/uri/lib/uri/generic.rb#L1053
[3] https://github.com/ruby/rubygems/blob/master/lib/rubygems/remote_fetcher.rb#L148
[4] https://www.rfc-editor.org/rfc/rfc2396#section-5.2
[5] https://www.rfc-editor.org/rfc/rfc2396#appendix-C