Refactor tidy_unzip()

[jennybc]

Fixes #1961, closes #1962.

Analyzing the above gave me a new/different understanding of common ZIP file organization structures. I think I've got a better heuristic for setting exdir.

[jennybc]

In tests/testthat/ref/README.md I now have a good origin story for the different ways ZIP archives are organized:

• "Explicit parent" is what you get with zip -r foo-explicit-parent.zip foo/.Note that foo/ is there, in its own right:

  ~/rrr/usethis/tests/testthat/ref % unzip -Z1 foo-explicit-parent.zip
  foo/
  foo/file.txt
  

• "Implicit parent" is what you get with zip -r foo-implicit-parent.zip foo/*. This is the form reported in use_zip error in unzipping directory with a different name from the zipfile #1961. Note that the original existence of foo/ is evident from the filepath but also only from the filepath:

  ~/rrr/usethis/tests/testthat/ref % unzip -Z1 foo-implicit-parent.zip
  foo/file.txt
  

• "No parent" is what you get with (cd foo && zip -r ../foo-no-parent.zip .). No evidence that file.txt ever lived in foo/:

  ~/rrr/usethis/tests/testthat/ref % unzip -Z1 foo-no-parent.zip
  file.txt