Fix import of bigquery transfer config location

[modular-magician]

Fix the custom import code for bigquery transfer config, and make it import the location properly.

Fixes hashicorp/terraform-provider-google#12011

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
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Release Note Template for Downstream PRs (will be copied)

bigquerydatatransfer: fixed a bug when importing 'location' of 'google_bigquery_data_transfer_config'

Derived from GoogleCloudPlatform/magic-modules#8768