RFC: retire pre-scan plugin stage

[pombredanne]

We could consider retiring pre-scan.

1. this is seldom used
2. this forces walking the whole codebase multiple times
3. this would be replace by a way to plugin in the filtering of the codebase, so that this can be walked only once
4. facets and classification can be made into regular scans that other can depend on

The plugins that use it:

• https://github.com/aboutcode-org/scancode-plugins/blob/9fcc50df36afdeb7b876eea4ec46fd2d2332772f/misc/scancode-ignore-binaries/setup.py#L48
• builtins classify = summarycode.classify:FileClassifier
• builtins facet = summarycode.facet:AddFacet
• builtins ignore = scancode.plugin_ignore:ProcessIgnore

The benefits:

• simpler code
• enable faster codebase walking

Based on a chat with @armijnhemel

[armijnhemel]

This would potentially positively impact #2725