provide debug and diagnostics rather than just saying Processed 0 items (0 created, 0 updated, 0 failed, 0 ignored)

Problem/Motivation

Re-running previously successful import not successful:

[notice] Processed 0 items (0 created, 0 updated, 0 failed, 0 ignored) - done with 'bol_csv_import'

No reason given why nothing was processed.

I even renamed my import file so that the migrate tool wouldn't find the import file - but it didn't even say file not found - just the same Processed 0 items (0 created, 0 updated, 0 failed, 0 ignored) - so I don't even know that it found my file second time around!

Drupal watchdog logs show nothing.

Note - same issue raised on migrate_source_csv: https://www.drupal.org/project/migrate_source_csv/issues/3268560 →
I'm not sure yet whether the migrate_tools or migrate_source_csv should deal with this issue.

Steps to reproduce

Followed these steps for my import file here: https://www.drupal.org/docs/8/api/migrate-api/migrate-source-plugins/mig... →

I could give my csv but I'm not sure this is necessary. In any case, I think logging and more informational output should be provided to give details of success and failure of import as it runs to help people troubleshoot.

Such diagostic and informational logs could have also helped similar cases:
- https://www.drupal.org/project/migrate_plus/issues/2940236 →
- https://www.drupal.org/project/migrate_source_csv/issues/2922510 →
- https://www.drupal.org/project/migrate/issues/1805440 →

Proposed resolution

Provide diagnostics and log output to show what's happening in the migrate, as above.

Remaining tasks

See 'Proposed resolution'

User interface changes

See 'Proposed resolution'

API changes

Not yet known.

Data model changes

Not expected. This request is about logging what's going on with existing code, not changing that code.