Abstract filesystem operations in doc_gen.py (#182)

- Update collect_snippets method to pass fs parameter to imported functions
- Ensures collect_snippets() and collect_snippet_files() use filesystem abstraction

Author: cpyle0819

aws_doc_sdk_examples_tools/doc_gen.py

@@ -75,13 +75,14 @@ def collect_snippets(
     ):
         prefix = prefix or ""
         snippets_root = snippets_root or self.root
-        snippets, errs = collect_snippets(snippets_root)
+        snippets, errs = collect_snippets(snippets_root, fs=self.fs)
         collect_snippet_files(
             self.examples.values(),
             prefix=prefix,
             snippets=snippets,
             errors=errs,
             root=self.root,
+            fs=self.fs,
         )
         self.snippets = snippets
         self.errors.extend(errs)