Define a JSON schema for the package JSON, and clarify some naming rules

docs/guide/packages.rst

 Package names aren't a complete free-for-all. Package names must follow a set
 of specific rules:
 
-- Package names may consist of ASCII, lowercase characters, digits,
-  underscores (``_``), hyphens (``-``), and periods (``.``).
+- Package names may consist of a subset of ASCII including lowercase
+  characters, digits, underscores (``_``), hyphens (``-``), and periods
+  (``.``).
 
   .. note::
     Different filesystems differ in their handling of filenames. Some platforms

package.json5

-{
-    "$schema": "what",
-    name: 'dds',
-    namespace: 'dds',
-    version: '0.1.0',
-    depends: {
-        spdlog: '1.4.2',
-        'ms-wil': '2019.11.10',
-        'range-v3': '0.10.0',
-        'nlohmann-json': '3.7.1',
-        'neo-sqlite3': '0.2.2',
-        'neo-fun': '0.1.0',
-        'semver': '0.2.1',
-        pubgrub: '0.2.0',
-        json5: '0.1.2',
-    },
-    test_driver: 'Catch-Main',
-}

package.jsonc

+{
+    "$schema": "./res/package-schema.json",
+    "name": "dds",
+    "version": "0.1.0",
+    "namespace": "dds",
+    "depends": {
+        "spdlog": "1.4.2",
+        "ms-wil": "2019.11.10",
+        "range-v3": "0.10.0",
+        "nlohmann-json": "3.7.1",
+        "neo-sqlite3": "0.2.2",
+        "neo-fun": "0.1.0",
+        "semver": "0.2.1",
+        "pubgrub": "0.2.0",
+        "json5": "0.1.2"
+    },
+    "test_driver": "Catch-Main"
+}

res/package-schema.json

+{
+    "type": "object",
+    "description": "DDS Package Manifest",
+    "additionalProperties": false,
+    "patternProperties": {
+        "^\\$": {}
+    },
+    "required": [
+        "name",
+        "version",
+        "namespace"
+    ],
+    "properties": {
+        "name": {
+            "type": "string",
+            "description": "The name of the package. Must be a valid Semantic Version string.",
+            "pattern": "^[a-z][a-z0-9_]*((\\.|-)[a-z0-9_]+)*$"
+        },
+        "version": {
+            "type": "string",
+            "pattern": "^(0|[1-9]\\d*)\\.(0|[1-9]\\d*)\\.(0|[1-9]\\d*)(?:-((?:0|[1-9]\\d*|\\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\\.(?:0|[1-9]\\d*|\\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\\+([0-9a-zA-Z-]+(?:\\.[0-9a-zA-Z-]+)*))?$",
+            "description": "The version of the package. Required",
+            "default": "0.1.0"
+        },
+        "namespace": {
+            "type": "string",
+            "description": "The package's namespace. Must be a valid string.",
+            "pattern": "^[a-z][a-z0-9_]*((\\.|-)[a-z0-9_]+)*$"
+        },
+        "$schema": {
+            "type": "string",
+            "description": "JSON schema tag. Ignored by dds."
+        },
+        "depends": {
+            "type": "object",
+            "patternProperties": {
+                "^[a-z][a-z0-9_]*((\\.|-)[a-z0-9_]+)*$": {
+                    "type": "string",
+                    "description": "The version of the dependency. Must be a valid Semantic Version string",
+                    "pattern": "^(0|[1-9]\\d*)\\.(0|[1-9]\\d*)\\.(0|[1-9]\\d*)(?:-((?:0|[1-9]\\d*|\\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\\.(?:0|[1-9]\\d*|\\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\\+([0-9a-zA-Z-]+(?:\\.[0-9a-zA-Z-]+)*))?$"
+                }
+            }
+        },
+        "test_driver": {
+            "type": "string",
+            "default": "Catch-Main",
+            "enum": [
+                "Catch-Main",
+                "Catch"
+            ]
+        }
+    }
+}