Extends variable-built `source` resolution to two path-construction forms
that were silently dropped: `source "$(dirname "$VAR")/lib/x.sh"` (command
substitution in the source argument) and `source "$VAR/../lib/x.sh"` (a
`..` suffix when the leading var is a tracked var_bases entry).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>