Rename `package-test-dir' to `package-test-data-dir'

It is more descriptive
Use private-symbol naming convention.

Symbols private to packages should be named `foo--func' to distinguish them from
publicly-facing symbols.
Fix `package--file-from-tar'

The code was not copied over correctly
Add and test `commentary' slot to `package-desc' struct
Create `package--archive-contents-filename', the name of archive-contents files

This is so the specific file name is not hard-coded.
Strip comment characters from commentary string.
Don't create a `package-test-user-dir' outside of `with-package-test'

This results in it not being cleaned up.
package-x: Fix syntax errors
Remove special case for a package named "package"

It did nothing more than write the same file to the same location a second time.
Tests for package-x