pathauditor: make _checkfs_exists a static method
This fixes the bug detected by pytype where the auditor
used in vfs.py may be a no-op auditor (vfs.py, line 398),
which doesn't have the _checkfs_exists method.
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
end_of_line = lf
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true
end_of_line = lf
[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false
end_of_line = lf