(Sources-3) oak_scan::set_package_sources()

[DavisVaughan]

Progress towards #1234
Branched from #1287

This PR teaches oak_scan how to update an installed Package's files and scripts from an external on disk directory.

The idea is that some external source provider will return a flat directory of R files that correspond to the source files for a Package. The new oak_scan::set_package_sources() helper iterates over that directory, collecting the R files (but not reading them!), sorting and splitting them in accordance with collate, and stashes them in files and scripts of the Package.

Caution

This PR has an important side effect. Since package.files() now live in a totally separate directory outside any LiveRoot, the File::package backpointer is now a load bearing feature.

Consider File::root(). It switches on self.package(). If there is no package backpointer, it would go through either:

• root_by_file(), which looks inside LiveRoot::Workspace and LiveRoot::Library root paths for the file's path, neither of which would hold it
• root_by_path(), which is for orphaned files, and also uses paths contained within WorkspaceRoots

So we need to instead detect self.package() and go through root_by_package(), which doesn't do a path check, it just detects that the Package backpointer is contained within a LiveRoot (by equality, not by path containment). We already do all this, it is just important to point it out now.

File::imports() is similar

[lionel-]

Not important to support right now or ever, but in principle a package might install .R files in inst and source them at run time?