Simplify Pharo code importing infrastructure

[jecisc]

Here is a step at simplifying the Pharo code importers.

• Merge some classes (classes with only one subclass)
• Remove dead code
• Factorize some code
• Move classes in the right packages
• Rename the importers to have more explicit names
• Make File structure importer inherit from object

There are still improvements we can do but this PR is already big enough :)