walker: Decouple WalkFunc to reduce walker dependencies

[radeksimko]

Currently the module walker/indexer has many dependencies:

terraform-ls/internal/terraform/module/walker.go

Lines 63 to 74 in 40a6ae8

	func NewWalker(fs ReadOnlyFS, ps PathStore, ms *state.ModuleStore, pss *state.ProviderSchemaStore, js job.JobStore, tfExec exec.ExecutorFactory) *Walker {
	return &Walker{
	pathStore: ps,
	fs: fs,
	modStore: ms,
	jobStore: js,
	schemaStore: pss,
	tfExecFactory: tfExec,
	logger: discardLogger,
	ignoreDirectoryNames: skipDirNames,
	}
	}

This makes it harder to change anything that involves the walker and harder to test.

We can extract the logic which happens when a module is found as WalkFunc and have that function alone depend on Job Scheduler etc.

terraform-ls/internal/terraform/module/walker.go

Lines 199 to 200 in 40a6ae8

	// TODO: decouple into its own function and pass as WalkFunc to NewWalker
	// this could reduce the amount of args in NewWalker

[radeksimko]

https://github.com/hashicorp/terraform-ls/compare/f-walker-refactoring represents a very rough idea of how this could be implemented, but it still needs some further thinking about watcher-related indexing logic, which is likely to change as part of #867 - specifically in the sense that walker should no longer need to be explicitly adding each walked path to the watcher as LSP-based watching supports wildcards.

With that in mind I'm putting this on hold until #790 is finally reviewed & merged and revisit with #867 in mind.

[radeksimko]

Currently blocked by #953

[radeksimko]

Also it would be best to first finish & merge #924

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