* Remove old Singularity versions, add version 3.1.1
* Recent versions of Singularity build with go rather than autotools:
update dependencies and build logic
* Move old Singularity versions to new singularity-legacy package
which uses the autotools build system
* Some binaries built by Singularity need to be run as root: include
a script that the user can run after the Spack Singularity install
to change these permissions.
* The Singularity go build expects to work with a custom stage
directory relative to GOPATH: override the stage phase to create
this expected path.
* Update Singularity install config to point to Spack-installed
makesquashfs dependency