spack external find: change default behavior (#29031)

See https://github.com/spack/spack/issues/25353#issuecomment-1041868116

This commit changes the default behavior of
```
$ spack external find
```
from searching all the possible packages Spack knows about to
search only for the ones tagged as being a "build-tool".

It also introduces a `--all` option to restore the old behavior.

lib/spack/spack/cmd/external.py

@@ -39,8 +39,17 @@ def setup_parser(subparser):
         '--scope', choices=scopes, metavar=scopes_metavar,
         default=spack.config.default_modify_scope('packages'),
         help="configuration scope to modify")
+    find_parser.add_argument(
+        '--all', action='store_true',
+        help="search for all packages that Spack knows about"
+    )
     spack.cmd.common.arguments.add_common_arguments(find_parser, ['tags'])
     find_parser.add_argument('packages', nargs=argparse.REMAINDER)
+    find_parser.epilog = (
+        'The search is by default on packages tagged with the "build-tools" or '
+        '"core-packages" tags. Use the --all option to search for every possible '
+        'package Spack knows how to find.'
+    )
 
     sp.add_parser(
         'list', help='list detectable packages, by repository and name'
@@ -48,6 +57,14 @@ def setup_parser(subparser):
 
 
 def external_find(args):
+    # If the user didn't specify anything, search for build tools by default
+    if not args.tags and not args.all and not args.packages:
+        args.tags = ['core-packages', 'build-tools']
+
+    # If the user specified both --all and --tag, then --all has precedence
+    if args.all and args.tags:
+        args.tags = []
+
     # Construct the list of possible packages to be detected
     packages_to_check = []
 
@@ -64,9 +81,10 @@ def external_find(args):
         # Since tags are cached it's much faster to construct what we need
         # to search directly, rather than filtering after the fact
         packages_to_check = [
-            spack.repo.get(pkg) for pkg in
-            spack.repo.path.packages_with_tags(*args.tags)
+            spack.repo.get(pkg) for tag in args.tags for pkg in
+            spack.repo.path.packages_with_tags(tag)
         ]
+        packages_to_check = list(set(packages_to_check))
 
     # If the list of packages is empty, search for every possible package
     if not args.tags and not packages_to_check:

lib/spack/spack/test/cmd/external.py

@@ -124,7 +124,7 @@ def test_find_external_cmd_not_buildable(
 
 def test_find_external_cmd_full_repo(
         mutable_config, working_env, mock_executable, mutable_mock_repo):
-    """Test invoking 'spack external find' with no additional arguments, which
+    """Test invoking 'spack external find --all' with no additional arguments
     iterates through each package in the repository.
     """
 
@@ -134,7 +134,7 @@ def test_find_external_cmd_full_repo(
     prefix = os.path.dirname(os.path.dirname(exe_path1))
 
     os.environ['PATH'] = ':'.join([os.path.dirname(exe_path1)])
-    external('find')
+    external('find', '--all')
 
     pkgs_cfg = spack.config.get('packages')
     pkg_cfg = pkgs_cfg['find-externals1']