Fix importer not finding existing users if their username is different

[m1r0]

The importer looks for existing users by username only. This causes an issue when the user has created a user, on a new instance, with the same email but with a different username.

Proposed Changes

Fix the importer to look for existing users by their email as well.

Testing Instructions

1. Go to Sensei LMS -> Tools -> Export Content and do an export.
2. Note some of the usernames and emails that are exported. For example username: test, email: test@example.com.
3. Create a new WP instance.
4. Create a user with an email of some of the exported users but use a different username. For example username: handsome_potato, email: test@example.com.
5. Go to Sensei LMS -> Tools -> Import Content and import the data.
6. There should be no error saying "Sorry, that email address is already used".
7. The data should be associated correctly to the user.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #7361 (7033f01) into trunk (7a66daa) will increase coverage by 0.00%.
Report is 6 commits behind head on trunk.
The diff coverage is 38.46%.

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk    #7361   +/-   ##
=========================================
  Coverage     50.73%   50.73%           
- Complexity    11144    11147    +3     
=========================================
  Files           613      613           
  Lines         47037    47044    +7     
  Branches        404      404           
=========================================
+ Hits          23862    23866    +4     
- Misses        22848    22851    +3     
  Partials        327      327           

Files	Coverage Δ
...des/data-port/class-sensei-data-port-utilities.php	89.95% <100.00%> (+0.17%)	⬆️
...es/blocks/course-theme/class-course-navigation.php	0.00% <0.00%> (ø)

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[renatho]

Looks good and works well!