Consider unpinning `SourceFile` and `io.spine.code.fs` from `base`

[alexander-yevsyukov]

Follow-up to the moves in #938–#941; requires an API decision, so filed separately.

io.spine.code.fs (AbstractFileName, AbstractSourceFile, FsObject) and io.spine.code.proto.SourceFile are imported only by tooling (ToolBase; SourceFile also by the dormant mc-js), yet they cannot move today:

1. FileSet.topLevelMessages() and FileSet.findMessageTypes(Predicate) construct SourceFile, and FileSet is runtime-essential — KnownTypes.load() → FileSet.load() runs at application startup.
2. io.spine.code.proto.FileName (runtime: used by MessageFile, Type, FileSet) extends code.fs.AbstractFileName.

If the two FileSet methods are relocated to ToolBase (e.g. as extension functions over FileSet), SourceFile, AbstractSourceFile, and FsObject could follow to ToolBase. AbstractFileName stays in base regardless, because of FileName.

This changes the public API of a runtime type (FileSet), so it needs a design decision rather than a mechanical move. Pre-2.0 is the cheapest time if we want it at all.

[alexander-yevsyukov]

This is not of a priority now. We might re-visit this issue in major release. So, I'm postponing this until v3.0