Store PklProject annotations in the project metadata (#708)

Write annotations to project metadata, and provide them to pkl-doc\

The following annotations have meaning for pkl-doc:
* `@Unlisted`: hide package from documentation site
* `@Deprecated`: add deprecated information

pkl-commons-test/src/main/files/packages/deprecated@1.0.0/deprecated@1.0.0.json

@@ -0,0 +1,33 @@
+{
+  "schemaVersion": 1,
+  "packageUri": "package://localhost:0/deprecated@1.0.0",
+  "name": "deprecated",
+  "version": "1.0.0",
+  "packageZipUrl": "https://localhost:0/deprecated@1.0.0/deprecated@1.0.0.zip",
+  "dependencies": {},
+  "packageZipChecksums": {
+    "sha256": "$computedChecksum"
+  },
+  "sourceCode": "https://example.com/deprecated",
+  "documentation": "https://example.com/deprecated-docs",
+  "license": "UNLICENSED",
+  "authors": [
+    "deprecated@example.com"
+  ],
+  "issueTracker": "https://example.com/deprecated/issues",
+  "annotations": [
+    {
+      "type": "PObject",
+      "classInfo": {
+        "moduleName": "pkl.base",
+        "class": "Deprecated",
+        "moduleUri": "pkl:base"
+      },
+      "properties": {
+        "since": "1.0.0",
+        "message": "don't use",
+        "replaceWith": null
+      }
+    }
+  ]
+}

pkl-commons-test/src/main/files/packages/deprecated@1.0.0/package/deprecated.pkl

@@ -0,0 +1,9 @@
+/// A module from a deprecated package
+module deprecated.deprecated
+
+/// Old foo
+@Deprecated { replaceWith = "bar" }
+foo: Int = 1
+
+/// New bar
+bar: Int = 2

pkl-commons-test/src/main/files/packages/unlisted@1.0.0/package/unlisted.pkl

@@ -0,0 +1,5 @@
+/// No docs are actually generated for this module
+module unlisted.unlisted
+
+/// 1
+foo: Int = 1

pkl-commons-test/src/main/files/packages/unlisted@1.0.0/unlisted@1.0.0.json

@@ -0,0 +1,29 @@
+{
+  "schemaVersion": 1,
+  "packageUri": "package://localhost:0/unlisted@1.0.0",
+  "name": "unlisted",
+  "version": "1.0.0",
+  "packageZipUrl": "https://localhost:0/unlisted@1.0.0/unlisted@1.0.0.zip",
+  "dependencies": {},
+  "packageZipChecksums": {
+    "sha256": "$computedChecksum"
+  },
+  "sourceCode": "https://example.com/unlisted",
+  "documentation": "https://example.com/unlisted-docs",
+  "license": "UNLICENSED",
+  "authors": [
+    "unlisted@example.com"
+  ],
+  "issueTracker": "https://example.com/unlisted/issues",
+  "annotations": [
+    {
+      "type": "PObject",
+      "classInfo": {
+        "moduleName": "pkl.base",
+        "class": "Unlisted",
+        "moduleUri": "pkl:base"
+      },
+      "properties": {}
+    }
+  ]
+}