Filter paths before parsing and lowering AST when possible

### Describe the issue:

When we filter out a file through --filter-paths, Slither still wastes time analyzing that file, and only skips returning the results for it.
Is this not a bug?

### Code example to reproduce the issue:

Not needed.

### Version:

0.10.2

### Relevant log output:

_No response_

Trace the --filter-paths handling through Slither's analysis pipeline, focusing on where files are parsed and their ASTs lowered. Verify the current behavior and add coverage showing that excluded files are skipped before those stages, while retained files continue to produce results.

Written by the indexing model from the issue text.