pw_presubmit: Path filtering; callable _Check
pw_presubmit: Path filtering; callable _Check

- Use re.search to filter paths instead of re.fullmatch. ^ and $ can be
  used to match the whole string.
- Filter paths using Path.as_posix() so that they are the same across
  Windows, macOS, and Linux.
- Make _Check objects callable. That makes it possible to reuse
  functions with @filter_paths and apply different filters.

Change-Id: I66e020aabe66c3f3ff50f167f54b28e2949b276f

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed