Fixes#10769
This updates the .headers property to include header subdirectories
for Python and Eigen (as is recommended by these packages).
#10623 updated the default behavior of .headers.directories to
exclude subdirectories (since this can cause clashes with system
headers). This broke some packages which depended on the old behavior
of .headers.directories: for example if you had
<package-prefix>/include/subdir/ex1.h, .headers.directories would
include <package-prefix>/include/subdir.
af4a36c4d9