perf data: Support single perf.data file directory

perf data: Support single perf.data file directory

Support directory output that contains a regular perf.data file, named
"data". By default the directory is named perf.data i.e.
	perf.data
	└── data

Most of the infrastructure to support a directory is already there. This
patch makes the changes needed to support the format above.

Presently there is no 'perf record' option to output a directory.

This is preparation for adding support for putting a copy of /proc/kcore in
the directory.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lore.kernel.org/lkml/20191004083121.12182-5-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>