This patch centralizes the logic to pick up code fragments embedded in
*.t script, in order to:

  - apply checking with patterns in check-code.py on such embedded
    code fragments

    Now, check-code.py completely ignores embedded code
    fragments. I'll post another patch series to check them.

  - replace similar code path in contrib/import-checker.py

    Current import-checker.py has problems below. Fixing each of them
    is a little difficult, because parsing logic and pattern strings
    are tightly coupled.

    - overlook (or mis-detect) the end of inline script in doctest
      style

      8a8dd6e4a97a fixed a part of this issue, but not enough.

    - it overlooks inline script in doctest style at the end of file
      (and ignores invalid un-closed heredoc at the end of file, too)

    - it overlooks code fragment in styles below
      - "python <<EOF" (heredoc should be "cat > file <<EOF" style)
      - "cat > foobar.py << ANYLIMIT" (...