https://w3id.org/cwl/CommonWorkflowLanguage.yml returns 404

In several places inside the `cwltool` codebase there are refeences to the https://w3id.org/cwl/CommonWorkflowLanguage.yml link, both in YAML files (e.g. in the [extensions](https://github.com/common-workflow-language/cwltool/blob/0e2ced53f8cd2652c4117af697fe2f5c6c94fcb7/cwltool/extensions.yml#L5)) and in the Python code (e.g. [here](https://github.com/common-workflow-language/cwltool/blob/0e2ced53f8cd2652c4117af697fe2f5c6c94fcb7/cwltool/process.py#L222)). However, this file does not exist (the link returns a `404` error).

This affects, for example, `schema_salad` parsers, which try to resolve the import without success. Simply redirecting the link to a specific version of the `CommonWorkflowLanguage.yml` file could be not enough, as different `extension.yml` files import the same link, even if they must refer to different versions of the file.

A discussion is needed on how to proceed.