Prepend ./ for files specified as CLI args

The get_module_qualname_from_path() function called by the node
visistor expects that all files are explicitly named with a
"head" and "tail" which are path delimiters to denote where the
file is within a python project.

However, if someone uses the command line and simply asks bandit
to scan dummy.py in the current working directory, it will be
missing the explicit "./" prefix in order for get_module_qualname_from_path
to run and determine the module fully qualified name from the path.

So this fix simply prepends a dot and delimiter to explicitly denote
a file in the current working directory as given from the CLI.

Fixes #907

Signed-off-by: Eric Brown <eric_wade_brown@yahoo.com>

bandit/core/manager.py

@@ -249,7 +249,7 @@ def discover_files(self, targets, recursive=False, excluded_paths=""):
                     excluded_path_globs,
                     enforce_glob=False,
                 ):
-                    files_list.add(fname)
+                    files_list.add(os.path.join(".", fname))
                 else:
                     excluded_files.add(fname)