v0.16.2 (2021-05-22)

• Major performance improvement for spack load and other commands. (#23661)
• spack fetch is now environment-aware. (#19166)
• Numerous fixes for the new, clingo-based concretizer. (#23016, #23307,
  #23090, #22896, #22534, #20644, #20537, #21148)
• Supoprt for automatically bootstrapping clingo from source. (#20652, #20657
  #21364, #21446, #21913, #22354, #22444, #22460, #22489, #22610, #22631)
• Python 3.10 support: collections.abc (#20441)
• Fix import issues by using __import__ instead of Spack package import. (#23288, #23290)
• Bugfixes and --source-dir argument for spack location. (#22755, #22348, #22321)
• Better support for externals in shared prefixes. (#22653)
• spack build-env now prefers specs defined in the active environment. (#21642)
• Remove erroneous warnings about quotes in from_sourcing_files. (#22767)
• Fix clearing cache of InternalConfigScope. (#22609)
• Bugfix for active when pkg is already active error. (#22587)
• Make SingleFileScope able to repopulate the cache after clearing it. (#22559)
• Channelflow: Fix the package. (#22483)
• More descriptive error message for bugs in package.py (#21811)
• Use package-supplied autogen.sh. (#20319)
• Respect -k/verify-ssl-false in _existing_url method. (#21864)