Bugfix external find --all for libraries (#31186)

* Fixed a bug in the 'external find --all' command where the call failed
to find packages by both executable and library. The bug was that the
call `path.all_packages()` incorrectly turned the variable
`packages_to_check` into a generator rather than keeping it a list.
Thus the second call to `detection.by_library` had no work to do.

* Fixed the help message for the find external and compiler commands as
well as others that used the `scopes_metavar` field to define where
the results should be stored in configuration space.  Specifically,
the fact that configuration could be added to the environment was not
mentioned in the help message.

lib/spack/spack/cmd/external.py

@@ -125,7 +125,7 @@ def external_find(args):
 
     # If the list of packages is empty, search for every possible package
     if not args.tags and not packages_to_check:
-        packages_to_check = spack.repo.path.all_packages()
+        packages_to_check = list(spack.repo.path.all_packages())
 
     detected_packages = spack.detection.by_executable(
         packages_to_check, path_hints=args.path)

lib/spack/spack/config.py

@@ -110,7 +110,7 @@
 
 #: metavar to use for commands that accept scopes
 #: this is shorter and more readable than listing all choices
-scopes_metavar = '{defaults,system,site,user}[/PLATFORM]'
+scopes_metavar = '{defaults,system,site,user}[/PLATFORM] or env:ENVIRONMENT'
 
 #: Base name for the (internal) overrides scope.
 overrides_base_name = 'overrides-'