Probably not for this pull request, but is it worth considering supporting standard colon/semi-colon separated paths for these env vars?  So that you can have a hierarchy of hooks?  For example

```bash
export SGTK_HOOK_PATH="a:b:c"
```
with a setting value of ```{$SGTK_HOOK_PATH}/hookname.py``` would match the first of these that exist:
- a/hookname.py
- b/hookname.py
- c/hookname.py