On Wednesday, 13 November 2013 at 04:35:12 UTC, Jesse Phillips wrote:

I don't see why we need such sub-directories. The language doesn't seem important to me.

I also place the source under /usr/src since they actually aren't headers/import files.

Mostly hygiene concerns. One of reasons why C headers are put into /usr/include is that it is default include location and it allows to to directly hit "#include <libheader>" after installing a package, with no explicit paths. Having similar standard location for D library modules is desired but FHS mandates it to be subdirectory of /usr/include (yes, those are import files for D). Making common subdirectory to avoid mixing with C stuff in filesystem is obvious next step.