Allow synchronization against the same local Drupal email account

Problem/Motivation

If a normal Drupal user exists, a o365 login should merge against the existing user with the same email address.

This issue has been addressed in the 3.0.x branch, but is still active in 2.0.5.

Steps to reproduce

Create a local Drupal user with a specific email, attempt to login using o365 against a user with the same email will cause a duplicate account to be created with the same email (leaving the database in an inconsistent state) or the following exception will be thrown if they happen to use their email as their username:

Drupal\externalauth\Exception\ExternalAuthRegisterException: User could not be registered. There is already an account with username "first.lastname@example.com" in Drupal\externalauth\ExternalAuth->register()

Proposed resolution

Address the if condition to check the email when the o365 account doesn't exist.

Remaining tasks

Provide issue fork/patch.

User interface changes

N/A