Cleanup of code inconsistencies with guidelines/conventions

[André Veríssimo]

While looking at our current code base I noticed some small inconsistencies:

• Use of double colon :: instead of importing (lifecycle, stringr, purrr, cli, rlang, etc..)
• test files not following convention
  • test-<filename>.R
    • For example, in test-metadata.R there are a bunch of xportr_label tests
      • it should only test xportr_metadata