Add package-map support for cross-repo SCIP navigation

- Add --package-map CLI option to specify namespace-to-package mappings
- Update symbol format to include package coordinates for cross-repo nav
- Skip external_symbols for packages that will be indexed separately
- This enables Sourcegraph to resolve cross-repo references and show
  documentation from the external package's index

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package-map.edn

@@ -0,0 +1,4 @@
+;; Package map for cross-repository SCIP navigation
+;; Maps namespace prefixes to package@version coordinates
+
+{"scip-clojure" "io.github.ajet/scip-clojure@main"}