Investigate new CodeDepends package

[krlmlr]

https://cran.r-project.org/web/packages/CodeDepends/index.html

https://cran.r-project.org/web/packages/CodeDepends/vignettes/intro.html

[wlandau-lilly]

For remake, it seems like CodeDepends could help solve #91 and #130. There is some really neat functionality, and it may or may not be worth changing the behavior of remake::code_dependencies().