[ssci] Refactor ParseDir to call helper functions

The goal is to implement changes so that //tools/licenses/licenses.py
can support README.chromium files that have multiple dependencies
described in the one file.

This CL refactors ParseDir to separate file parsing and metadata
processing, but still limited to one dependency per file. A follow-up
CL will then be created to support multiple dependencies per file.

Bug: b:299237794
Change-Id: I5f1d435a7c110f6d902d34f40083b1c68e0d6cff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4844917
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Anne Redulla <aredulla@google.com>
Cr-Commit-Position: refs/heads/main@{#1193394}