Problem/Motivation
When running and debugging migrations from Drupal 7, it becomes tedious and frustrating to have to repeatedly set the database connection up.
Proposed resolution
Allow the source database to be set up via settings by looking for a database with the key name migrate_drupal_source. If it is present, skip the "Source database" section of migrate_drupal_ui's CredentialForm.
Remaining tasks
User interface changes
If a database is already set up, the "Source database" section of Drupal\migrate_drupal_ui\Form\CredentialForm is omitted.
API changes
The database key migrate_drupal_source will now have a significant meaning.
Data model changes
None.
Release notes snippet
TBD
