Report warning when using the package without invoking AddServices

[kzu]

This source generator is quite magical, but it still can't inject itself in the
service collection without users invoking the provided
ServiceCollection.AddServices() extension method. This has tripped a
a new user
in the past, which is not a great user experience.

We should make every attempt to detect this situation, so the idea is to:

1. Detect if users have access to IServiceCollection in the current project: it may be that
   they just added the package to get the [Service] attribute for a library. Nothing to do
   in that case.
2. Detect if users actually invoke IServiceCollection members in the current project, more
   or less flexibly (i.e. don't verify much where specifically, since the DI library can be used
   in multiple contexts). If they do, then it's an indication that there should be a call to the
   AddServices extension method too somewhere. As a hint, report the location on the
   first invocation to a service collection method (or extension method).
3. Report as a warning, since we can determine this wrong, depending on whether users
   are initializing services some other way or indirectly. This allows them to ignore the warning
   using the regular configuration mechanisms for analyzers and diagnostics.