Refactor pkgmgr self-install handling, add pip-enabled Python env to flake, and fix Nix/pip integration

- Rename requirements.txt to _requirements.txt to avoid unintended self-install via PythonInstaller.
- Update flake.nix to provide Python with pip in both devShell and runtime closure.
- Generate a wrapper for pkgmgr that uses the pip-enabled interpreter.
- Adjust NixFlakeInstaller to correctly handle old profile removal and install outputs.
- Update run_command to support string commands and allow_failure.
- Improve integration test by adding nix profile cleanup and debugging output.

Reference: https://chatgpt.com/share/69345df2-a960-800f-8395-92a7c3a6629f

flake.nix

@@ -1,7 +1,7 @@
 {
   description = "Nix flake for Kevin's package-manager tool";
 
-    nixConfig = {
+  nixConfig = {
     extra-experimental-features = [ "nix-command" "flakes" ];
   };
 
@@ -23,9 +23,16 @@
       # Dev shells: nix develop .#default (on both architectures)
       devShells = forAllSystems (system:
         let
-          pkgs   = nixpkgs.legacyPackages.${system};
+          pkgs = nixpkgs.legacyPackages.${system};
+
+          # Base Python interpreter
           python = pkgs.python311;
-          pypkgs = pkgs.python311Packages;
+
+          # Python env with pip + pyyaml available, so `python -m pip` works
+          pythonEnv = python.withPackages (ps: with ps; [
+            pip
+            pyyaml
+          ]);
 
           # Be robust: ansible-core if available, otherwise ansible.
           ansiblePkg =
@@ -34,8 +41,7 @@
         in {
           default = pkgs.mkShell {
             buildInputs = [
-              python
-              pypkgs.pyyaml
+              pythonEnv
               pkgs.git
               ansiblePkg
             ];
@@ -46,48 +52,59 @@
         }
       );
 
-packages = forAllSystems (system:
-  let
-    pkgs   = nixpkgs.legacyPackages.${system};
-    python = pkgs.python311;
-    pypkgs = pkgs.python311Packages;
+      packages = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          python = pkgs.python311;
 
-    # Optional: ansible mit in den Closure nehmen
-    ansiblePkg =
-      if pkgs ? ansible-core then pkgs.ansible-core
-      else pkgs.ansible;
-  in
-  rec {
-    pkgmgr = pkgs.stdenv.mkDerivation {
-      pname   = "package-manager";
-      version = "0.1.1";
+          # Runtime Python for pkgmgr (with pip + pyyaml)
+          pythonEnv = python.withPackages (ps: with ps; [
+            pip
+            pyyaml
+          ]);
 
-      src = ./.;
+          # Optional: include Ansible in the runtime closure
+          ansiblePkg =
+            if pkgs ? ansible-core then pkgs.ansible-core
+            else pkgs.ansible;
+        in
+        rec {
+          pkgmgr = pkgs.stdenv.mkDerivation {
+            pname   = "package-manager";
+            version = "0.1.1";
 
-      # Nix soll *kein* configure / build ausführen (also auch kein make)
-      dontConfigure = true;
-      dontBuild     = true;
+            src = ./.;
 
-      # Wenn du Python/Ansible im Runtime-Closure haben willst:
-      buildInputs = [
-        python
-        pypkgs.pyyaml
-        ansiblePkg
-      ];
+            # Nix should not run configure / build (no make)
+            dontConfigure = true;
+            dontBuild     = true;
 
-      installPhase = ''
-        mkdir -p "$out/bin"
-        cp main.py "$out/bin/pkgmgr"
-        chmod +x "$out/bin/pkgmgr"
-      '';
-    };
+            # Runtime deps: Python (with pip) + Ansible
+            buildInputs = [
+              pythonEnv
+              ansiblePkg
+            ];
 
-    # default package just points to pkgmgr
-    default = pkgmgr;
-  }
-);
+            installPhase = ''
+              mkdir -p "$out/bin"
 
+              # Wrapper that always uses the pythonEnv interpreter, so
+              # sys.executable -m pip has a working pip.
+              cat > "$out/bin/pkgmgr" << EOF
+#!${pythonEnv}/bin/python3
+import runpy
+if __name__ == "__main__":
+    runpy.run_module("main", run_name="__main__")
+EOF
 
+              chmod +x "$out/bin/pkgmgr"
+            '';
+          };
+
+          # default package just points to pkgmgr
+          default = pkgmgr;
+        }
+      );
 
       # Apps: nix run .#pkgmgr / .#default
       apps = forAllSystems (system: