feat: Allow pre-loading modules

Add option to preload modules.
Preloading modules allows to render members of objects
that originate from other packages than their parent.
Direct members of modules must be listed in `__all__`.

This option typically allows to render aliases,
by collecting their parent module (package)
which allows to resolve these aliases.

Issue mkdocstrings/mkdocstrings#503: mkdocstrings/mkdocstrings#503
PR #60: mkdocstrings/python#60

Author: viktorlashchuk

docs/schema.json

@@ -262,6 +262,14 @@
               "markdownDescription": "https://mkdocstrings.github.io/python/usage/#globallocal-options",
               "enum": ["brief", "source"],
               "default": "brief"
+            },
+            "preload_modules": {
+              "title": "Pre-load modules. It permits to resolve aliases pointing to these modules (packages), and therefore render members of an object that are external to the given object (originating from another package).",
+              "markdownDescription": "https://mkdocstrings.github.io/python/usage/#globallocal-options",
+              "type": "array",
+              "items": {
+                "type":"string"
+              }
             }
           },
           "additionalProperties": false

src/mkdocstrings_handlers/python/handler.py

@@ -99,6 +99,7 @@ class PythonHandler(BaseHandler):
         "members": None,
         "filters": ["!^_[^_]"],
         "annotations_path": "brief",
+        "preload_modules": None,
     }
     """
     Attributes: Headings options:
@@ -150,6 +151,16 @@ class PythonHandler(BaseHandler):
     Attributes: Additional options:
         show_bases (bool): Show the base classes of a class. Default: `True`.
         show_source (bool): Show the source code of this object. Default: `True`.
+        preload_modules (list[str] | None): Pre-load modules that are
+            not specified directly in autodoc instructions (`::: identifier`).
+            It is useful when you want to render documentation for a particular member of an object,
+            and this member is imported from another package than its parent.
+
+            For an imported member to be rendered, you need to add it to the `__all__` attribute
+            of the importing module.
+
+            The modules must be listed as an array of strings. Default: `None`.
+
     """  # noqa: E501
 
     def __init__(
@@ -235,7 +246,10 @@ def collect(self, identifier: str, config: Mapping[str, Any]) -> CollectorItem:
                 modules_collection=self._modules_collection,
                 lines_collection=self._lines_collection,
             )
-            try:
+            try:  # noqa: WPS229 we expect one type of exception, and want to fail on the first one
+                for pre_loaded_module in final_config.get("preload_modules") or []:
+                    if pre_loaded_module not in self._modules_collection:
+                        loader.load_module(pre_loaded_module)
                 loader.load_module(module_name)
             except ImportError as error:
                 raise CollectionError(str(error)) from error