Account for child files in Rmd

[zkamvar]

This will address #497 by allowing child file changes to cause the rebuild of a parent file. This is done in three steps:

1. The accounting for child documents now occurs right after the intial hashes are taken.
2. We search for child documents only in R Markdown documents
3. We take the hash of the combined hashes of the child documents and the parent file using rlang::hash() so if one of those hashes changes, then the parent file is rebuilt, but if none change, then the parent file remains.

In addition, this modifies serve() and build_lesson() to take into account the situation when someone passes a child file to build_lesson() so that the correct parent file is found to rebuild.

To test this locally, with a lesson, you can install it with:

remotes::install_github("carpentries/sandpaper#499")

To test this in a live lesson, you can use this in your config.yaml

sandpaper: carpentries/sandpaper@use-child-files-for-db

To run the development tests, you can use the PR commands from usethis inside the sandpaper repository:

library("usethis")
pr_fetch(499)
devtools::test()

When you are done, run this to get back to the main branch.

pr_finish()