(cherry picked from commit 6f4ba636f1e563167e6456d42c9fb6f65c9ca504)
This allows cargo metadata to operate on it without adding the source files to the build. (A choice which will save a few rebuilds of the manual) (cherry picked from commit 1779295f3e13cc15f8422d52a3753bb927ac8fa7)
(cherry picked from commit b9150e82be712b2cd82d689f0f26c2a133564701)