Fix `add_function_hyperlinks()`

[jacobbien]

The function add_function_hyperlinks() should not add hyperlinks for functions that have <pkg>:: before them, unless <pkg> is the name of the package currently being defined. To see why this is an issue, notice the hyperlinks in rmarkdown::render() and devtools::document() in create-litr.html:

These will link to the definition of litr::render() and litr::document(). We should also create a test in create-litr.Rmd to demonstrate that rmarkdown::render() (and devtools::document() are not hyperlinked).