Implement monorepo sub-module support

[heartwilltell]

Phase 6: Polish — RFC #218

Support packages defined in subdirectories of a monorepo. Module paths include the subdirectory (github.com/user/monorepo/libs/utils), and tags use path prefixes (libs/utils/v1.0.0).

Closes #223.

Monorepo Support (from RFC)

• A run.toml in a subdirectory defines a sub-module
• Module path includes the subdirectory: github.com/user/monorepo/libs/utils
• Tags for sub-modules use the path prefix: libs/utils/v1.0.0

Acceptance Criteria

• Module paths with 4+ segments treated as sub-module
• Tag discovery filters for path-prefixed tags (subdir/v1.0.0)
• Archive download extracts only the relevant subdirectory
• run.toml in subdirectory defines the sub-module's metadata
• Import resolution maps sub-module path to correct cache location
• Tests for sub-module discovery, downloading, and import resolution

Files

• Modified: src/modfetch.zig, src/modcache.zig, src/resolve.zig

Dependencies

• Implement GitHub tag discovery via git ls-remote #299 (tag discovery)
• Implement source archive download from GitHub #301 (archive download)
• Extend import resolution to recognize external module paths #312 (import resolution)

Part of the Package Manager RFC.