Export pkg_file_lua for package author to use

[cderv]

This will close #1903

Once merged we could use it in other packages like bookdown and pagedown

[yihui]

Looks good overall. Thanks!

[cderv]

Review was taken into account and test is passing. I'll merge. You can modify if I missed anything.

I'll be porting this to other packages using lua filter

[yihui]

Sounds good. Thanks!

[cderv]

So at the end, this is not as useful as I thought because it won't work as expected in another package development workflow.

• It work well from inside rmarkdown package, because devtools::load_all will add a shim for system.files
• but this shim does not apply when system.files is used in another function from a package. That means using rmarkdown::pkg_file_lua inside bookdown will not allow to use the system.file shim. That means, when doing load_all on bookdown, it won't find the lua files. You need to install the development package first!

It is a bit annoying while developping and you are used to devtools workflow. testthat is using load_all also. So test won't work with this function.

However, if you use last version of testit no issue... because it install the package before testing 😉

Anyway, I note this here so that we don't forget what happens. I don't know if the shims in pkgload could work differently - I'll may open an issue there.