Rename `GitlabImport::Logger` `import_source` key to `import_type`

What does this MR do?

To better match with Project properties, rename GitlabImport::Logger import_source key to import_type.

In the Project model Project#import_type is used to save the source provider, like 'github', 'gitlab', 'bitbucket', etc. While, the Project#import_source is used to refer the project path in the source provider, for instance when importing the "Ruby on Rails" project from GitHub, the import_type will be github and the import_source will be rails/rails.

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How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
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• This change is backwards compatible across updates, or this does not apply.

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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