Investigate need for file_public_base_url

[paul-m]

Fixes [#4205]

NOTE: Do Not Merge Just Yet.

This PR requires a change to the DKAN DDev add-on. See GetDKAN/ddev-dkan#52

Describe your changes

• Adds tests for ResourceImporter, kernel and functional.
• Performs minor refactoring of ResourceImporter:
  • Short-circuits unneeded else statements.
  • Injects services explicitly.
• Adds steps to SampleContentCommandsTest to prove that the Drush commands work without setting file_public_base_url.

Discoveries

• Drupal\common\Util\DrupalFiles uses the deprecated system_retrieve_file() function, which goes away in Drupal 11.
• DrupalFiles also does not handle file wrappers properly, leading to an inability to write a kernel test. (Kernel tests use in-memory vfsStream file wrappers.)

Follow-up recommendations:

• Refactor DrupalFiles to play nice with vfsStream and also to not use system_retrieve_file().
• Turn Drupal\Tests\harvest\Functional\Transform\ResourceImporterTest into a kernel test.
• Investigate whether we actually need DrupalFiles, since it's only used in Drupal\harvest\Transform\ResourceImporter and ResourceLocalizer service.

QA Steps

• Stand up a local build with the special settings config
• Create sample content
• Remove the config
• Remove sample content
• Create sample content again

Checklist before requesting review

If any of these are left unchecked, please provide an explanation

• I have updated or added tests to cover my code
• I have updated or added documentation