Customizable detect func for finder, fix SearchSingleTree

What does this MR do?

• Add customizable Detect function to finder
• Remove Parser field from finder.File; this wasn't used since !136 (merged)
• Add unit tests to finder, to emulate the search needed for gemnasium-maven and gemnasium-python
• Fix search mode SearchSingleTree, to be used by gemnasium-maven

2 possible next steps:

• use this new generic finder in gemnasium-maven and gemnasium-python, along with custom detect functions
• or work on !134 (merged) to detect projects instead of files

What are the relevant issue numbers?

gitlab-org/gitlab#263441 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer