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If the client is sticking with a composer build, you can keep the nested docroot, and can create the site without an upstream: Creating a Pantheon website with no defined upstream
If sticking with the nested docroot, you should rename the docroot directory to web, since that is what pantheon expects. Add a symlink from docroot to web to account for any composer config that includes the docroot path.
Do whatever it takes to get the old code in the new repository. (and maybe add more details here)
Set up the pantheon.yml file
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Available drush versions are at: https://pantheon.io/docs/drush-versions/#configure-drush-version
Code Things
Rename the docroot directory to web, since that is what pantheon expects.
Add a symlink from docroot to web to account for any composer config that includes the docroot path.
Sync the files directory
You can use these instructions for Manually transferring drupal files with rsync.
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