New get-deps command: infer PyPI depedencies from mkdocs.yml

The user story is that the following command should let you "just build" any MkDocs site:

    pip install $(mkdocs get-deps) && mkdocs build

This cross-references 2 files:

* mkdocs.yml - `theme`, `plugins`, `markdown_extensions`
* projects.yaml - a registry of all popular MkDocs-related projects and which entry points they provide - downloaded on the fly

-and prints the names of Python packages from PyPI that need to be installed to build the current MkDocs project

pyproject.toml

@@ -44,6 +44,7 @@ dependencies = [
     "typing_extensions >=3.10; python_version < '3.8'",
     "packaging >=20.5",
     "mergedeep >=1.3.4",
+    "platformdirs >=2.2.0",
     "colorama >=0.4; platform_system == 'Windows'",
 ]
 [project.optional-dependencies]
@@ -63,6 +64,7 @@ min-versions = [
     "typing_extensions ==3.10; python_version < '3.8'",
     "packaging ==20.5",
     "mergedeep ==1.3.4",
+    "platformdirs ==2.2.0",
     "colorama ==0.4; platform_system == 'Windows'",
     "babel ==2.9.0",
 ]