fall back to sequential installation for extensions with unknown dependencies when using --parallel-extensions-install

[boegel]

This avoids a hard crash that occurs currently when trying to use --parallel-extensions-install --experimental --robot and an easyconfig that includes Python packages as extensions.

For example:

NotImplementedError: Don't know how to determine required dependencies for extension 'setuptools'

This occurs because the PythonPackage easyblock does not define the required_deps method, and hence the default implementation from the Extension easyblock is used.

The changes include here include:

• changing the default implementation of required_deps to just return None;
• fall back to sequential installation for an extension with unknown dependencies, that is wait until all preceding extensions have been installed first (since the order in the list of extensions should then be honored).

The added test verifies that this is working as expected.

[branfosj]

lgtm