This particular invocation of 'sort' is expensive (it may amount to 30%
of the total ckati execution time for the clean build).
Test: verify that the result of running 'm nothing' is unchanged,
verify that the result of running 'm -j deps-license
PROJ_PATH=packages/app/% DEP_PATH=external/%' is unchanged.
Change-Id: If1cfddd4dee24559a26ecceebdf03cf49cc5a367