We have two representations of a source. Once used for sorting, which
should not depend on the source's state, but solely on its static
information, like remotes. Another one used for error and informational
messages, which should properly inform about the exact state of the
source when the message is printed.

This commit makes the latter be the default implementation of `to_s`, so
that error and informational messages are more accurate by default.

https://github.com/rubygems/rubygems/commit/b5f2b88957

When printing sources inside these error messages, it's useful to only
consider the current state of the source. For example, when requiring
`bundler/setup`, the source shouldn't be configured to be able to hit
the network, so the error message should only mention "locally installed
gems" to make that more clear.

https://github.com/rubygems/rubygems/commit/30eb14f853