v2.0 beta release

This is the v 2.0 beta version. Verified in the internal production environment.

Features:

• Inclusion check for "hdrs" and private headers
• The new "load" statement to write and use extension
• Visibility is private for all targets by default and supports "legacy public" to fit the old codebase.
• A better method for append/prepend config items
• Concurrency maven downloading
• Many performance improvements, code quality improvements include refactoring, ci, testing, docs, lint, and bugfix