refactor: ast.Call should be in main task package

[pd93]

Call doesn't belong in the taskfile/ast package. It's not a part of the Taskfile schema. It was previously used, but has since been refactored out. Since a Call is something that the tool does, I feel it should be a part of the main task package.

In order to do this, there are a couple of other small refactors that need to be done so that we don't get circular dependency issues:

1. Compiler needs moving into the main task package instead of being in its own internal package. Historically, Compiler was an interface with multiple implementations, but this is no longer the case, so I don't think there is a need for package anymore.
2. FindMatchingTasks also needs to be moved from the taskfile/ast package into the main task package, but this makes sense to me anyway. The taskfile/ast package should be for the AST/paring code only, not for logic. I have moved this to a method on the Executor for now.