Add `IN_PKGDOWN_PKG`?

[maelle]

When running, pkgdown sets the IN_PKGDOWN variable to TRUE.

pkgdown/R/pkgdown.R

Line 15 in f3ec6e2

IN_PKGDOWN = "true",

Could this behavior be extended to create a variable called IN_PKGDOWN_PKG that'd contain the name of the package currently being documented. Like roxygen2::roxy_meta_get("current_package").

Use case: when you set some specific behavior for your package in the pkgdown website for reasons but don't want that behavior to apply when reverse dependencies of your package build their pkgdown website. https://github.com/cynkra/dm/blob/604a275e496b8cd082db612197fe963f13e46295/R/pkgdown.R#L10

cynkra/dm#2194

I'll open a PR for this.