Python: Move metrics-related API to LegacyPointsTo module

Gets rid of the `getMetrics` methods on the `Function`, `Class`, and
`Module` classes. To access the metrics, one must first import the
`LegacyPointsTo` module, and then either change the type to
`{Function,Class,Module}Metrics` or cast to the appropriate type.

python/ql/lib/LegacyPointsTo.qll

@@ -34,6 +34,7 @@ import semmle.python.types.Exceptions
 import semmle.python.types.Properties
 import semmle.python.types.Descriptors
 import semmle.python.SelfAttribute
+import semmle.python.Metrics
 
 /**
  * An extension of `ControlFlowNode` that provides points-to predicates.

python/ql/lib/python.qll

@@ -14,7 +14,7 @@ import semmle.python.Patterns
 import semmle.python.Keywords
 import semmle.python.Comprehensions
 import semmle.python.Flow
-import semmle.python.Metrics
+private import semmle.python.Metrics
 import semmle.python.Constants
 import semmle.python.Scope
 import semmle.python.Comment

python/ql/lib/semmle/python/Class.qll

@@ -147,9 +147,6 @@ class Class extends Class_, Scope, AstNode {
   /** Gets a base of this class definition. */
   Expr getABase() { result = this.getParent().getABase() }
 
-  /** Gets the metrics for this class */
-  ClassMetrics getMetrics() { result = this }
-
   /**
    * Gets the qualified name for this class.
    * Should return the same name as the `__qualname__` attribute on classes in Python 3.

python/ql/lib/semmle/python/Function.qll

@@ -84,9 +84,6 @@ class Function extends Function_, Scope, AstNode {
   /** Gets the name used to define this function */
   override string getName() { result = Function_.super.getName() }
 
-  /** Gets the metrics for this function */
-  FunctionMetrics getMetrics() { result = this }
-
   /**
    * Whether this function is a procedure, that is, it has no explicit return statement and always returns None.
    * Note that generator and async functions are not procedures as they return generators and coroutines respectively.

python/ql/lib/semmle/python/Module.qll

@@ -86,9 +86,6 @@ class Module extends Module_, Scope, AstNode {
     result = this.getName().regexpReplaceAll("\\.[^.]*$", "")
   }
 
-  /** Gets the metrics for this module */
-  ModuleMetrics getMetrics() { result = this }
-
   string getAnImportedModuleName() {
     exists(Import i | i.getEnclosingModule() = this | result = i.getAnImportedModuleName())
     or