Issue 13766: Fix broken ddoc build for std/regex/package.d.

Partial fix for: https://issues.dlang.org/show_bug.cgi?id=13766

The trouble is that `std.regex` got split into `std/regex/package.d` and `std/regex/internal/*`, but the makefile rules for generating ddocs didn't get updated to handle `std/regex/package.d` correctly.

Note that this PR is not a complete fix, because the navigation bar on the left is implemented in the dlang.org repo (why, oh why) so that will have to be separately fixed.