Moving csharp_library to have a single file in returned DefaultInfo

[jimevans]

This allows dependent rules to easily use csharp_library rules for things like copying files to another output location. Having multiple files returned makes such a thing difficult.

[j3parker]

@omsmith: Jim and I talked about this in Slack. It's really convenient if DefaultInfos files is simple (in this case just the DLL). It makes using our targets from things like genrule simpler. Some notes:

• We want to ut things like the pdb and transitive DLLs into runfiles (for bazel run/bazel test)
• I'll make csharp_binary and csharp_nunit_test do the same thing.
• More advanced rules (e.g. nuget package creation) should ignore DefaultInfo and use the C# providers instead
• refout isn't needed in runfiles ever
• We probably want to populate runfiles with transitive deps for libraries too, even though they can't be directly run (other rules might look at their runfiles, though.) I'll deal with that.