rustpkg now makes source files that it checks out automatically read-only, and stores
them under build/.
Also, refactored the `PkgSrc` type to keep track of separate source and destination
workspaces, as well as to have a `build_workspace` method that returns the workspace
to put temporary files in (usually the source, sometimes the destination -- see
comments for more details).
Closes#6480
As per rustpkg.md, rustpkg now builds in a target-specific
subdirectory of build/, and installs libraries into a target-specific
subdirectory of lib.
Closes#8672
