command: add spack find --format

- spack find --format allows you to supply a format string and have specs
  output in a more machine-readable way, without dedcoration

e.g.:

    spack find --format "{name}-{version}-{hash}"
    autoconf-2.69-icynozk7ti6h4ezzgonqe6jgw5f3ulx4
    automake-1.16.1-o5v3tc77kesgonxjbmeqlwfmb5qzj7zy
    bzip2-1.0.6-syohzw57v2jfag5du2x4bowziw3m5p67
    ...

or:

    spack find --format "{hash}"
    icynozk7ti6h4ezzgonqe6jgw5f3ulx4
    o5v3tc77kesgonxjbmeqlwfmb5qzj7zy
    syohzw57v2jfag5du2x4bowziw3m5p67
    ...

This is intended to make it much easier to script with `spack find`

lib/spack/spack/cmd/__init__.py

@@ -200,6 +200,20 @@ def gray_hash(spec, length):
     return colorize('@K{%s}' % h)
 
 
+def display_formatted_specs(specs, format_string, deps=False):
+    """Print a list of specs formatted with the provided string.
+
+    Arguments:
+        specs (list): list of specs to display.
+        deps (bool): whether to also print dependencies of specs.
+    """
+    for spec in specs:
+        print(spec.format(format_string))
+        if deps:
+            for depth, dep in spec.traverse(depth=True, root=False):
+                print("   " * depth, dep.format(format_string))
+
+
 def display_specs(specs, args=None, **kwargs):
     """Display human readable specs with customizable formatting.
 

lib/spack/spack/cmd/find.py

@@ -10,6 +10,7 @@
 
 import spack.environment as ev
 import spack.repo
+import spack.cmd as cmd
 import spack.cmd.common.arguments as arguments
 from spack.cmd import display_specs
 from spack.util.string import plural
@@ -33,11 +34,18 @@ def setup_parser(subparser):
                               const='paths',
                               help='show paths to package install directories')
     format_group.add_argument(
+        "--format", action="store", default=None,
+        help="output specs with the specified format string")
+
+    # TODO: separate this entirely from the "mode" option -- it's
+    # TODO: orthogonal, but changing it for all commands that use it with
+    # TODO: display_spec is tricky. Also make -pd work together properly.
+    subparser.add_argument(
         '-d', '--deps',
         action='store_const',
         dest='mode',
         const='deps',
-        help='show full dependency DAG of installed packages')
+        help='output dependencies along with found specs')
 
     arguments.add_common_arguments(
         subparser, ['long', 'very_long', 'tags'])
@@ -143,6 +151,26 @@ def decorator(spec, fmt):
     return decorator, added, roots, removed
 
 
+def display_env(env, args, decorator):
+    tty.msg('In environment %s' % env.name)
+
+    if not env.user_specs:
+        tty.msg('No root specs')
+    else:
+        tty.msg('Root specs')
+        # TODO: Change this to not print extraneous deps and variants
+        display_specs(
+            env.user_specs, args,
+            decorator=lambda s, f: color.colorize('@*{%s}' % f))
+        print()
+
+    if args.show_concretized:
+        tty.msg('Concretized roots')
+        display_specs(
+            env.specs_by_hash.values(), args, decorator=decorator)
+        print()
+
+
 def find(parser, args):
     q_args = query_arguments(args)
     results = args.specs(**q_args)
@@ -168,25 +196,13 @@ def find(parser, args):
         results = [x for x in results if x.name in packages_with_tags]
 
     # Display the result
-    if env:
-        tty.msg('In environment %s' % env.name)
-
-        if not env.user_specs:
-            tty.msg('No root specs')
-        else:
-            tty.msg('Root specs')
-            # TODO: Change this to not print extraneous deps and variants
-            display_specs(
-                env.user_specs, args,
-                decorator=lambda s, f: color.colorize('@*{%s}' % f))
-        print()
-
-        if args.show_concretized:
-            tty.msg('Concretized roots')
-            display_specs(
-                env.specs_by_hash.values(), args, decorator=decorator)
-            print()
-
-    tty.msg("%s" % plural(len(results), 'installed package'))
-
-    display_specs(results, args, decorator=decorator, all_headers=True)
+    if args.format:
+        cmd.display_formatted_specs(
+            results, args.format, deps=(args.mode == "deps"))
+    elif args.json:
+        cmd.display_specs_as_json(results, deps=(args.mode == "deps"))
+    else:
+        if env:
+            display_env(env, args, decorator)
+        tty.msg("%s" % plural(len(results), 'installed package'))
+        display_specs(results, args, decorator=decorator, all_headers=True)