fix(install): follow same-host absolute path imports in transitive scan
Imports like `import "/src/foo.ts"` (common on deno.land/x and esm.sh
CDNs) fell through the BFS without being queued, so a transitive
remote graph that used absolute paths skipped type coverage for those
modules. Treat any non-scheme-prefixed specifier beginning with `/` as
absolute-against-the-current-URL and resolve it via Url::join, the
same path `./` and `../` already use.