Refactor functions to be smaller and more modular

[AliSajid]

BiocCheck::BiocCheck() issues a NOTE for functions longer than 50 lines. Currently, 3 functions are longer than 50 lines. These functions should be refactored to be smaller and more modular.

The Note in question is:

* Checking function lengths...
    * NOTE: The recommended function length is 50 lines or less. There are 3 functions greater than 50 lines.

The functions in question are:

• investigate_target() #26
• investigate_signature() #27
• prepare_signature() #28

The Idea would be to encapsulate some of the logic that is common to these and other fucntions into unexported utility functions. This way, we can defer the error or check logic to these functions and keep the core functions more readable and concise.