Create a generic abstraction over all state entities

[dbanck]

• Part of Improve language extensibility #1614
• Depends on Split monolithic state into multiple entities #1626

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We need to introduce a new way to interacts with variable definition related things, after they've been removed from the Module struct. Access through the module store won't be possible anymore, so we need a new abstraction.

terraform-ls/internal/langserver/handlers/did_open.go

Lines 40 to 54 in 62b906e

	mod, err := svc.modStore.ModuleByPath(dh.Dir.Path())
	if err != nil {
	if state.IsModuleNotFound(err) {
	err = svc.modStore.Add(dh.Dir.Path())
	if err != nil {
	return err
	}
	mod, err = svc.modStore.ModuleByPath(dh.Dir.Path())
	if err != nil {
	return err
	}
	} else {
	return err
	}
	}

This will allow the LSP handlers (like did_open, did_change) to continue to work as before, but instead of accessing a module directly, they use the abstraction and the language ID to complete the operation, like adding an entity or checking for existence.