Treat scripts in R/ folder as collated

[lionel-]

Progress towards posit-dev/positron#14790
Progress towards #1338

Files in an R/ folder are now treated in an alphabetical collation, just like in packages. This means they can now see each other's functions.

The alphabetical order only matters for top-level expressions (eager context). For lazy contexts (like functions), the symbols resolve to the end-of-collation view, where all symbols from all files are defined.

Goals:

• Provide a conventional way of having interdependent files when we can't reliably detect the dependencies.
• Support auto-load mechanisms as in Shiny: https://shiny.posit.co/r/reference/shiny/1.7.5/loadsupport.html

This is not technically needed for the targets::tar_source() use case which we will support with a more precise mechanism in subsequent PRs.

Note that while this allows files in the R/ folder to see each other, this has no effect at the sourcing site, e.g. tar_source(), or the implicit call to loadSupport() made by Shiny. This is a different side of the problem to be tackled separately.

Positron Release Notes

New Features

• Files in an R/ folder are now treated in an alphabetical collation, as in packages (go to definition returns no definition found positron#14790). Concretely functions in sibling files can now see each other. This currently only affects the symbol navigation and renaming features.

Bug Fixes

• N/A