This directory contains trimmed-down versions of various modules that
remove a lot of dependencies. When unit tests are compiled, the
versions here have precedence over the versions in the main source
directories. However, the versions here can themselves be replaced by
files with the same name in the source directory of a particular unit
test.

That is, the precedence is:

(1) source directory of unit test
(2) unit_test_shr
(2) unit_test_replacements
(4) main source directories