While the new manifest.py style got introduced for freezing python code
into the resulting binary, the old way - where files and modules within
ports/*/modules where baked into the resulting binary - was still
supported via `freeze('$(PORT_DIR)/modules')` within manifest.py.
However behaviour changed for symlinked directories (=modules), as those
links weren't followed anymore.
This commit restores the original behaviour by explicitly following
symlinks within a modules/ directory
2f71d66ef7