Restructuring 'ALLOWED_SPDX_LICENSES'

.. to show license classifications via comments

Change-Id: Id18ee6a597457ecaaafbd24373d78aca09ba7058
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6055001
Commit-Queue: Jordan Brown <rop@google.com>
Reviewed-by: Jordan Brown <rop@google.com>
Reviewed-by: Rachael Newitt <renewitt@google.com>

metadata/fields/custom/license_allowlist.py

@@ -6,9 +6,15 @@
 # These licenses are used to verify that code imported to Android complies with
 # their licensing requirements. Do not add entries to this list without approval.
 # Any licenses added should be a valid SPDX Identifier. For the full list of
-# identifiers; see https://spdx.org/licenses/
+# identifiers; see https://spdx.org/licenses/.
+# Licenses below are grouped by restrictiveness level and then alphabetically.
 ALLOWED_SPDX_LICENSES = frozenset([
-    "APSL-2.0",
+    # other_ignorable.
+    "LZMA-SDK-9.22",
+    # permissive.
+    "GPL-2.0-with-classpath-exception",
+    "MIT-0",
+    # notice.
     "Apache-2.0",
     "BSD-2-Clause",
     "BSD-2-Clause-FreeBSD",
@@ -16,24 +22,22 @@ ALLOWED_SPDX_LICENSES = frozenset([
     "BSD-4-Clause",
     "BSD-4-Clause-UC",
     "BSD-Source-Code",
-    "GPL-2.0-with-classpath-exception",
+    "BSL-1.0",
+    "ICU",
+    "ISC",
     "MIT",
-    "MIT-0",
     "MIT-Modern-Variant",
-    "MPL-1.1",
-    "MPL-2.0",
     "NCSA",
     "OFL-1.1",
     "SGI-B-2.0",
+    "SunPro",
     "Unicode-3.0",
     "Unicode-DFS-2015",
     "Unicode-DFS-2016",
     "X11",
     "Zlib",
-    # Public Domain variants.
-    "ISC",
-    "ICU",
-    "LZMA-SDK-9.22",
-    "SunPro",
-    "BSL-1.0",
+    # reciprocal. TODO(b/385020146): Only allow for opensource projects.
+    "APSL-2.0",
+    "MPL-1.1",
+    "MPL-2.0",
 ])