Print an error and bail when using a vendor tarball for a library crate

Building a library-only crate against vendored dependencies is nonsense,
since the resulting package will not be able to ship anything.

rust2rpm/__main__.py

@@ -163,6 +163,10 @@ def main():
             log.error(f"This is not supported by rust2rpm; remove the '+{build_meta}' suffix.")
             sys.exit(1)
 
+        if vendor_tarball and not (package.is_bin() or package.is_cdylib()):
+            log.error(f"Building library-only crates with vendored dependencies is not supported.")
+            sys.exit(1)
+
         warn_if_package_uses_restrictive_dependencies(package)
 
         spec_contents = spec_render_crate(