libs: Do not rely on nftw for detecting the validity of plugin path

libs: Do not rely on nftw for detecting the validity of plugin path

test_plugin.c:test_create_all_from_dir with non existing path fails on
cygwin. An error is expected but bt_plugin_find_all_from_dir returns
success.

The cygwin implementation of nftw is not posix compliant regarding
returning ENOENT on non-existent path. [1] [2]

The cygwin implementation of nftw does not return ENOENT on non-existent
path. Instead, the non-existent path is passed through the fn function
(nftw_append_all_from_dir) and the flag is set to FTW_NS.

Since we are "discovering" plugins, nftw_append_all_from_dir does not
error out on FTW_NS and simply skip the file.

The proposed solution is to stat() the provided path and validate that
no errors occur before using nftw().

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/nftw.html
[2] https://cygwin.com/ml/cygwin/2019-06/msg00185.html

Change-Id: Ia7c88dbab5b16affbdb8ce1dbe261b03635727cc
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1520
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>