Packaging: Refactor the cpp libraries

This PR separates the core cpp packs into `codeql/cpp-queries` and
`codeql/cpp-all`.

There are very few lines of code changed. Almost all changes are moving
files around.

cpp/ql/lib/semmle/code/cpp/Struct.qll

@@ -0,0 +1,75 @@
+/**
+ * Provides classes for modeling `struct`s.
+ */
+
+import semmle.code.cpp.Type
+import semmle.code.cpp.Class
+
+/**
+ * A C/C++ structure or union. For example, the types `MyStruct` and `MyUnion`
+ * in:
+ * ```
+ * struct MyStruct {
+ *   int x, y, z;
+ * };
+ *
+ * union MyUnion {
+ *   int i;
+ *   float f;
+ * };
+ * ```
+ */
+class Struct extends Class {
+  Struct() { usertypes(underlyingElement(this), _, 1) or usertypes(underlyingElement(this), _, 3) }
+
+  override string getAPrimaryQlClass() { result = "Struct" }
+
+  override string explain() { result = "struct " + this.getName() }
+
+  override predicate isDeeplyConstBelow() { any() } // No subparts
+}
+
+/**
+ * A C/C++ struct that is directly enclosed by a function. For example, the type
+ * `MyLocalStruct` in:
+ * ```
+ * void myFunction() {
+ *   struct MyLocalStruct {
+ *     int x, y, z;
+ *   };
+ * }
+ * ```
+ */
+class LocalStruct extends Struct {
+  LocalStruct() { isLocal() }
+
+  override string getAPrimaryQlClass() { not this instanceof LocalUnion and result = "LocalStruct" }
+}
+
+/**
+ * A C/C++ nested struct. See 11.12. For example, the type `MyNestedStruct` in:
+ * ```
+ * class MyClass {
+ * public:
+ *   struct MyNestedStruct {
+ *     int x, y, z;
+ *   };
+ * };
+ * ```
+ */
+class NestedStruct extends Struct {
+  NestedStruct() { this.isMember() }
+
+  override string getAPrimaryQlClass() {
+    not this instanceof NestedUnion and result = "NestedStruct"
+  }
+
+  /** Holds if this member is private. */
+  predicate isPrivate() { this.hasSpecifier("private") }
+
+  /** Holds if this member is protected. */
+  predicate isProtected() { this.hasSpecifier("protected") }
+
+  /** Holds if this member is public. */
+  predicate isPublic() { this.hasSpecifier("public") }
+}