Add support for targets::tar_source() and sourceDir()

[lionel-]

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

Source effects can now target a directory instead of a single file, which covers the two idioms for loading a folder of scripts: targets::tar_source() and the sourceDir() example from ?source.

# _targets.R
library(targets)
tar_source()                 # defaults to files = "R"
tar_target(x, my_helper())   # goto-def lands in R/my_helper.R

The resolver expands the directory into one target per script, in list.files() order. Load order within the call is respected: code/b.R sees code/a.R but not the other way around. Combined with the previous PR this also means the sourced files see each other, through the sourcing file's scope, without relying on the R/ auto-collation of the first PR.

The PR also adds ad hoc support for sourceDir() with a total hack: any calls to sourceDir() with a resolvable string causes a Source effect. This is to handle the documented pattern in ?source that people are copying around: https://github.com/search?q=sourceDir+language%3AR&type=code. Ideally we'd infer the effects from the list.files() and source() calls. One day.

Positron Release Notes

New Features

• Added support for directory sourcing with targets::tar_source() and a sourceDir() call as documented in ?source (go to definition returns no definition found positron#14790).

Bug Fixes

• N/A

[lionel-]

Edit: See #1388 (comment)