demo: add gogen package_test comprehensive test suite

Add a comprehensive test suite for gogen package functionality,
covering 29 different test cases including:
- Package import and redefinition handling
- Built-in type methods and conversions
- Type declarations and method definitions
- Interface assignments and type switches
- Channel operations and concurrency primitives

This test file was converted from gogen's package_test.go to
demonstrate llgo's compatibility with gogen package manipulation.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: luoliwoshang