shell: move cyclic taskmap plugin into subdir

Problem: Builtin shell taskmap plugins are currently stored at the
top level in src/shell, but it would be better to keep these in a
subdirectory to avoid naming confusion.

Move the cyclic taskmap plugin into src/shell/taskmap.

Author: grondo

src/shell/Makefile.am

@@ -89,7 +89,7 @@ flux_shell_SOURCES = \
 	doom.c \
 	exception.c \
 	rlimit.c \
-	cyclic.c \
+	taskmap/cyclic.c \
 	signal.c \
 	files.c \
 	oom.c \