Toward #1311, we want a standalone is_likely_used function which would take a ToolSpec and ConfigSpec and basically just give the current implementation for Tool.is_used, i.e. the default implementation.

Tool.is_used would be refactored to use is_likely_used using self for the ToolSpec and self.config_spec for the ConfigSpec.

Tool.is_used, whose current implementation would serve as the basis for is_likely_used, currently uses Tool.is_declared_as_dep, which should be moved into ToolSpec so it is accessible from the ToolSpec arg. It also uses Tool.is_config_present which uses a Tool._is_config_spec_present helper method together with ConfigSpec from Tool.config_spec. We should move the Tool._is_config_spec_present method to ConfigSpec.is_present since it doesn't actually use the self arg in Tool at all. Then, is_likely_used can just use ConfigSpec.is_present directly from the ConfigSpec passed as an argument.

Lastly, Tool.is_used uses Tool.is_pre_commit_config_present. This, along with its dependent method Tool.get_pre_commit_repos should be moved into ToolSpec.

The ToolSpec implementation should be moved into a new module called usethis._tool.spec.

The new standalone is_likely_used method should be put into a new usethis._tool.heuristics module.

The new module layering for usethis._tool should be like the following:
layers =
all_
impl
base
heuristics
spec
config | pre_commit | rule