Make compatible with domain 2.x

Problem/Motivation

This module is blocking use of domain 2.x due to version constraint in composer.json (which might be there for a good reason).

Steps to reproduce

Attempt to upgrade domain to 2.x on site with this module in composer.json:

 - drupal/domain_sites is locked to version 1.0.0-alpha2 and an update of this package was not requested.
 - drupal/domain_sites 1.0.0-alpha2 requires drupal/domain ^1 -> found drupal/domain[dev-1.x, 1.0.0-alpha1, ..., 1.x-dev (alias of dev-1.x)] but it conflicts with your root composer.json require (^2.0).

Proposed resolution

Initially just allow both 1.x and 2.x of domain, get it installed, then do some testing.

Remaining tasks

Create and review patch.

User interface changes

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API changes

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Data model changes

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