deprecated get_configtool_variable and get_pkgconfig_variable

The get_variable method is able to do everything they do and more,
making it generally more useful. Let's tell people to stop using the old
ones.

docs/markdown/Reference-manual.md

@@ -2422,10 +2422,14 @@ an external dependency with the following methods:
    *(since 0.45.0)* A warning is issued if the variable is not defined,
    unless a `default` parameter is specified.
 
+   *(Deprecated since 0.56.0*) use `get_variable(pkgconfig : ...)` instead
+
  - `get_configtool_variable(varname)` *(since 0.44.0)*: gets the
    command line argument from the config tool (with `--` prepended), or,
    if invoked on a non config-tool dependency, error out.
 
+   *(Deprecated since 0.56.0*) use `get_variable(configtool : ...)` instead
+
  - `type_name()`: returns a string describing the type of the
    dependency, the most common values are `internal` for deps created
    with `declare_dependency()` and `pkgconfig` for system dependencies

docs/markdown/snippets/deprecate_old_variable_methods.md

@@ -0,0 +1,4 @@
+## Deprecate Dependency.get_pkgconfig_variable and Dependency.get_configtool_variable
+
+These have been replaced with the more versatile `get_variable()` method
+already, and shouldn't be used anymore.

mesonbuild/interpreter.py

@@ -459,6 +459,8 @@ class DependencyHolder(InterpreterObject, ObjectHolder):
     def name_method(self, args, kwargs):
         return self.held_object.get_name()
 
+    @FeatureDeprecated('Dependency.get_pkgconfig_variable', '0.56.0',
+                       'use Dependency.get_variable(pkgconfig : ...) instead')
     @permittedKwargs({'define_variable', 'default'})
     def pkgconfig_method(self, args, kwargs):
         args = listify(args)
@@ -470,6 +472,8 @@ class DependencyHolder(InterpreterObject, ObjectHolder):
         return self.held_object.get_pkgconfig_variable(varname, kwargs)
 
     @FeatureNew('dep.get_configtool_variable', '0.44.0')
+    @FeatureDeprecated('Dependency.get_configtool_variable', '0.56.0',
+                       'use Dependency.get_variable(configtool : ...) instead')
     @permittedKwargs({})
     def configtool_method(self, args, kwargs):
         args = listify(args)