Small refactoring
HTML fix
Rename variable
Add a separate report file with the list of all files
Store source files in a 2-level directory structure

To avoid having thousands of files dumped in the same directory.
Remove impossible TODO

Scope hooks are inherited by cloned interpreters (e.g. threads) and can't be
unregistered.
Add method to list the report names for an aggregator
Add a method to remove some of the keys from custom metadata
Add per-report and per-aggregate metadata support

This adds the scaffolding (class definition, load/save code, ...) without
providing the actual functionality.
Support adding per-report metadata during aggregation
Optionally add @INC value to global metadata
If present, use @INC value stored in metadata to shorten file names