I'm on mobile but here are a few pointers : you could crawl all the exported functions of the packages (there are some functions in devtools for parsing namespace files) and then search your code for every function.
here is a function that gets exported functions of a package I wrote a while ago https://github.com/s-fleck/testthis/blob/master/R/Test_coverage.R (get _pkg_functions)
searching the source code should be easy (list.files, readLines, grep). if you need some pointers here, look through the source of my (experimental) github package sifr