Make sure to actually use yarn to install the main-packages, otherwise the packages.json#resolutions property will not be used and @babel/helpers would not get overruled

Overruled @babel/helpers to fix how initializers play with decorated properties. Thus circumventing the imperformant and lengthy code being generated by babel in the non-legacy option.

packages/babel-helpers/test/fixtures/dependencies/basic/output.js

@@ -0,0 +1,5 @@
+function _$_basic_main() { return _$_basic_dependency(); }
+
+function _$_basic_dependency() {}
+
+_$_basic_main;