Blockify package_basic_fields.html

IDatasetForm plugins often want to add custom fields to the first page of the three-stage dataset creation form, but currently they have to copy the entire `package_basic_fields.html` into their extension because it doesn't contain any blocks. Add a block that can be overridden to add custom fields to the form, as is already done in `package_metadata_fields.html` (the third page of the dataset creation form).