Refactor built-in project creation by template to have its own import_type value
About
Project creation by in-built template and file-based import both use project.import_type of "gitlab_project".
Although the custom template feature does have its own import_type of "gitlab_custom_project_template".
Problem
Although File-based project import and project creation by in-built template leverage the same underlying functionality, they have different feature needs !174610 (comment 2266230688), including permission models #414046 (comment 1945586449).
Being unable/difficult to distinguish between the two different features is leading to bugs #414046 (comment 1939649453) and #500081 (comment 2283741776) and other difficulties !174610 (comment 2266369856).
Proposal
To match the custom template feature import_type of "gitlab_custom_project_template", refactor project creation by in-built template to have its own unique import_type of "gitlab_project_template".
Ensure we update these areas to check the new import_type:
Note also there may be some technical debt to remove as part of #500081 (comment 2343143210).