Keep Python Bytecode out of the Bundled Python Home (#16550)

* Keep Python Bytecode out of the Bundled Python Home

The bundled stdlib ships .pyc files that are stale against their sources:
they are compiled when the Python deps are installed, and the copy into the
app does not keep the sources' mtimes. CPython then recompiles every module
it imports and writes the result next to the source. A drag-installed macOS
app rewrites files inside its own bundle on first launch, so its signature
no longer verifies, and read-only installs (AppImage, Flatpak, Program Files)
recompile the same modules on every launch.

Point pycache_prefix at <data_dir>/python/pycache so the interpreter reads
and writes bytecode there. The bundle is never written, the cache survives
between launches, and the data directory is already a plugin read/write root.

* Drop Cached Bytecode Through the Interpreter in the Module Release Test

tests/slic3rutils/test_plugin_lifecycle.cpp

@@ -615,8 +615,22 @@ TEST_CASE("Python module release removes package submodules and owned sys.path",
 
     // Ensure the next import executes the new submodule rather than reusing a stale package child.
     write_package("'new'");
-    boost::system::error_code ec;
-    fs::remove_all(package_root / "__pycache__", ec);
+    {
+        // The rewrite can keep the old mtime and size, so drop the bytecode wherever the
+        // interpreter caches it.
+        PythonGILState gil;
+        REQUIRE(gil);
+        PyObjectPtr util(PyImport_ImportModule("importlib.util"));
+        REQUIRE(util);
+        for (const char* file : {"__init__.py", "sub.py"}) {
+            PyObjectPtr source(PyUnicode_DecodeFSDefault((package_root / file).string().c_str()));
+            REQUIRE(source);
+            PyObjectPtr cached(PyObject_CallMethod(util.get(), "cache_from_source", "O", source.get()));
+            REQUIRE(cached);
+            boost::system::error_code ec;
+            fs::remove(fs::path(PyUnicode_AsUTF8(cached.get())), ec);
+        }
+    }
 
     paths.clear();
     modules.clear();