How do I mark a variable private across imports in Gnu Make?

Question

Gnu make provides private variables for specific rules.

https://www.gnu.org/software/make/manual/html_node/Suppressing-Inheritance.html

How do I mark a variable private across an import, so that it is only defined in its file.mk context, but not in the main makefile:

# makefile
include file.mk
foo=biz  # this is the value in main make

# file.mk
foo=bar  # make this private; this is the value in file.mk recipes