Fix issue #65: Check source directories exist before writing to requirements

[jensens]

Summary

Fixes #65 - mxdev now checks if source directories exist before writing them to requirements-mxdev.txt, with different behavior based on offline mode.

Problem

When using mxdev -n (no-fetch) or in offline mode, source directories may not exist yet. Previously, mxdev would write references like -e ./sources/package to requirements-mxdev.txt regardless, causing pip install to fail with "directory does not exist" errors.

This breaks the mxmake two-stage installation workflow:

1. mxmake runs mxdev -n to generate requirements file
2. mxdev writes -e ./sources/package even though sources don't exist
3. pip install fails

Solution

Modified write_dev_sources() to check if source directories exist with different behavior based on mode:

Offline Mode (expected missing sources)

• Missing sources log WARNING
• Packages written as comments
• mxdev continues execution
• Warning message: "This is expected in offline mode. Run mxdev without -n and --offline flags to fetch sources."

Non-Offline Mode (unexpected missing sources = fatal error)

• Missing sources log ERROR
• Packages written as comments
• mxdev raises RuntimeError and exits
• Error message: "This indicates a failure in the checkout process. Run mxdev without -n flag to fetch sources."

This prevents silent failures when sources fail to checkout.

Changes

• Update write_dev_sources() to accept State parameter and check Path(package["path"]).exists()
• Track missing sources and raise RuntimeError in non-offline mode
• Use logger.error() for non-offline mode, logger.warning() for offline mode
• Write missing packages as # -e ./sources/package # mxdev: source not checked out
• Add comprehensive tests for both modes
• Update all existing tests to pass State parameter

Testing

✅ All write_dev_sources tests pass (7/7)

• Updated test: test_write_dev_sources_missing_directories - now uses offline mode explicitly
• New test: test_write_dev_sources_missing_directories_raises_error - verifies RuntimeError in non-offline mode
• Existing test: test_write_dev_sources_missing_directories_offline_mode - verifies offline-specific warning

Benefits

• ✅ Fixes mxmake two-stage installation workflow
• ✅ Prevents silent failures when checkout fails
• ✅ Clear distinction between expected (offline) and fatal (non-offline) missing sources
• ✅ Prevents pip install failures from non-existent source paths
• ✅ Users can see what packages would be installed (commented lines)
• ✅ Makes mxdev more defensive and robust