The term “crate root” usually designates the src/lib.rs or src/main.rs file, i.e. the module at the root of a crate. You’re talking about something else, i.e. whether the crate is being built as a dependency for another package or not. Therefore, I think the name crate_root is not suitable here.
I also want to emphasize the distinction between a crate and a package. A package can build several crates: a library, binaries, tests, examples, etc. Presumably, the condition should evaluate to true when building the library for use in a test, even if the library is not the “root crate” in this scenario.
You also mention Cargo workspaces. A workspace is comprised of packages, not of crates.
Considering all of this, I would suggest naming the configuration option something like root_package, main_package or target_package. Perhaps it would be interesting to have an option for the current workspace in addition to the former; the name I have in mind is workspace_member (when compiling dependencies, workspaces are not involved).
