Fall back to Xcode DerivedData when using --skip-build

[ky1ejs]

Summary

When --skip-build is used without --index-store-path, periphery only looks for the index store in its own DerivedData cache directory (~/Library/Caches/com.github.peripheryapp/DerivedData-*). If the build was performed by Xcode or xcodebuild directly (rather than by periphery), the index store lives in Xcode's default DerivedData location and periphery fails with Failed to find index datastore.

This is a common scenario in CI pipelines where the project is built in a prior step and periphery runs with --skip-build to avoid rebuilding.

This PR adds a fallback: when the index store isn't found in periphery's own cache, search ~/Library/Developer/Xcode/DerivedData/ for directories matching the project name, preferring the most recently modified one.

Changes

• Extracted the index store path lookup (Index.noindex/DataStore or Index/DataStore) into a reusable findIndexStorePath(in:) method
• Added findIndexStoreInDefaultDerivedData(projectName:) that searches Xcode's default DerivedData location
• indexStorePath(project:schemes:) now tries periphery's cache first, then falls back to Xcode's DerivedData before throwing

Reproduction

# Build with xcodebuild (not periphery)
xcodebuild -scheme MyScheme -destination 'platform=iOS Simulator,name=iPhone 16' build

# This fails without the fix:
periphery scan --skip-build --strict
# Error: Failed to find index datastore at path: ~/Library/Caches/com.github.peripheryapp/DerivedData-...

# Workaround (no longer needed with this fix):
periphery scan --skip-build --index-store-path "$(find ~/Library/Developer/Xcode/DerivedData -path '*/Index.noindex/DataStore' -type d | head -1)" --strict

Fixes #1082