Use per-pkg sub dirs in build directory

[gavanderhoorn]

This would make the build directory a bit less crowded, especially when workspaces contain a lot of packages.

The sub directories could contain the depends files, possibly the Makefile snippet for the package (with the top-level Makefile including all of them) and any other future pkg-specific build files.

Build output (p-code, generated headers, etc) should still go into the main build directory.