search: add scaffolding for structural search in searcher

Created by: rvantonder

This is only scaffolding changes for structural search code path in searcher:

• rename matcher.go, matcher_test.go to search_regex.go, search_regex_test.go resp.
• introduce structural_search.go (no functionality yet).
• branch logic for regex versus structural search in searcher.go

Test plan: None, those will come when I add structural search functionality.