Reorganize package work directories

We now put the work directories for each package into
${work_path}/build, so that they're nicely segregated from the other
top-level directories in the Buzzy cache path.
Stage is a prereq of package, not install

This patch fixes up the prereq relationship between the builder and
packager for a source package.  Before, we marked the "stage" action as
a prereq of both the "package" action and the "install" action, but it
only needs to be a prereq of "package".
Merge branch 'feature/repo' into develop

* feature/repo:
  Stage is a prereq of package, not install
  Reorganize package work directories
  Add buzzy package description
  Default value for `version` in git repositories
  git version strings
  Detect CMake packages
  Print non-verbose output when error occurs
  Create package destination directory
  Default verbosity for building packages
  Default built packages in filesystem repos
  Package management commands
  Packages no longer contain dependencies
  Repository registry
  Hide phases with no actions
  Repositories
  Cache values in YAML environments
  Nested YAML environments
  YAML environments
  Base paths for value sets