fix(run): run Python modules with relative imports with -m (#1338)

Run or Debug on a Python file started it as a script
(`python src/tasks/cli.py`), so a module of a package that imports
relatively (`from .models import TaskList`) always failed with
"attempted relative import with no known parent package".

A file with relative imports now runs as `python -m package.module`
from the directory above its top package (the enclosing directories
with `__init__.py`, read once and re-read when one is added or
removed), with a venv-relative interpreter resolved against the project
root. Files without relative imports still run as scripts.

src/features/run/pythonPackages.ts

@@ -0,0 +1,30 @@
+/**
+ * Which directories are Python packages (`__init__.py`), for running a module with relative
+ * imports as `python -m package.module`. Read once per directory.
+ */
+import { call, commands, isTauri } from "@/platform/ipc";
+
+const known = new Map<string, boolean>();
+
+export function invalidatePythonPackages() {
+  known.clear();
+}
+
+/** Whether `dir` is a package, when already read. */
+export function isPythonPackage(dir: string): boolean | undefined {
+  return known.get(dir);
+}
+
+const parent = (p: string) => (p.includes("/") ? p.slice(0, p.lastIndexOf("/")) || "/" : "");
+
+/** Reads the packages enclosing `path`'s directory, up to the first directory that isn't one. */
+export async function loadPythonPackages(path: string): Promise<void> {
+  if (!isTauri()) return;
+  for (let dir = parent(parent(path)); dir; dir = parent(dir)) {
+    if (!known.has(dir)) {
+      const kind = await call(commands.fsEntryKind(`${dir}/__init__.py`)).catch(() => null);
+      known.set(dir, kind === "file");
+    }
+    if (!known.get(dir) || dir === "/") return;
+  }
+}