started popsu_migrate module

web/modules/custom/popsu_migrate/config/install/migrate_plus.migration.d7_allpublicfiles.yml

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+# Every migration that references a file by Drupal 7 fid should specify this
+# migration as an optional dependency.
+id: d7_allpublicfiles
+label: Public files
+migration_group: d7_popsu
+audit: true
+migration_tags:
+  - Drupal 7
+  - Content
+  - Popsu
+  - Files
+
+source:
+  plugin: d7_pubic_file
+  batch_size: 5000
+  scheme: public
+  constants:
+    # The tool configuring this migration must set source_base_path. It
+    # represents the fully qualified path relative to which URIs in the files
+    # table are specified, and must end with a /. See source_full_path
+    # configuration in this migration's process pipeline as an example.
+    source_base_path: '../../../d7'
+process:
+  # If you are using this file to build a custom migration consider removing
+  # the fid field to allow incremental migrations.
+  # fid: fid
+  filename: filename
+  source_full_path:
+    -
+      plugin: concat
+      delimiter: /
+      source:
+        - constants/source_base_path
+        - filepath
+    -
+      plugin: urlencode
+  uri:
+    plugin: file_copy
+    source:
+      - '@source_full_path'
+      - uri
+  filemime: filemime
+  # No need to migrate filesize, it is computed when file entities are saved.
+  # filesize: filesize
+  status: status
+  # Drupal 7 didn't keep track of the file's creation or update time -- all it
+  # had was the vague "timestamp" column. So we'll use it for both.
+  created: timestamp
+  changed: timestamp
+  uid: uid
+destination:
+  plugin: entity:file

web/modules/custom/popsu_migrate/config/install/migrate_plus.migration_group.d7_popsu.yml

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+id: d7_popsu
+label: D7 Popsu
+description: Drupal 7 popsu content migration to Drupal 9.
+source_type: Drupal 7
+shared_configuration:
+  source:
+    key: d7
+dependencies:
+  enforced:
+    module:
+      - migrate_plus
+      - migrate_tools
+      - popsu_migrate