Rework logic for skipping .depend/.meta file read/stat/writes.

• Rename _SKIP_READ_DEPEND to _SKIP_DEPEND since it also avoids writing.
• This now uses .NOMETA to avoid reading any .meta files related to DEPENDOBJS. Objects not in OBJS/DEPENDOBJS may still have their .meta files read in if they are in the dependency graph.
• This also avoids statting .meta and .depend files in the META_MODE + -DNO_FILEMON case.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon