refactor(ivy): move the flat module index generator to its own package (#27743)

This commit moves the FlatIndexGenerator to its own package, in preparation
to expand its capabilities and support re-exporting of private declarations
from NgModules.

PR Close #27743

packages/compiler-cli/BUILD.bazel

@@ -26,6 +26,7 @@ ts_library(
         "//packages/compiler",
         "//packages/compiler-cli/src/ngtsc/annotations",
         "//packages/compiler-cli/src/ngtsc/diagnostics",
+        "//packages/compiler-cli/src/ngtsc/entry_point",
         "//packages/compiler-cli/src/ngtsc/metadata",
         "//packages/compiler-cli/src/ngtsc/shims",
         "//packages/compiler-cli/src/ngtsc/switch",