fix: update elpaca installer from v0.11 to v0.12

Elpaca renamed repos/ to sources/ and changed several internal symbols.
The old installer was cloning into repos/ while the new elpaca code
expected sources/, causing "Non-existant source dir" errors.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

fix: show also warnings, fix two outstanding warnings

fix: use system-type, not window-system, it's nil for daemon

fix: no symlink mode for windows

chore: move elpaca to elpaca-loader

chore: split to more el files