Addons having a requirements.txt under the doc folder, try to install these dependencies

### Problem description

NOTE: This issue is present on 1.1.3 but works fine in dev. Will the fix be backported?

Here is an example addon which shows this behavior: https://github.com/MarcBresson/FreeCAD-Parametric-Export

here the [exact revision](https://github.com/MarcBresson/FreeCAD-Parametric-Export/tree/f51edae9ae8fff0a682a9bf1082044d09c06eff8) in case of an update of the addon.

### Workbench affected?

None

### Steps to reproduce

When you try to install the addon, FC will throw a warning:

```
This addon requires Python packages that are not installed, and cannot be installed automatically. To use this addon you must install the following Python packages manually:
* sphinx-copybutton
* sphinx
* sphinx-github-changelog
* furo
To ignore this error and install anyway, press OK.
```

However, the mentioned packages are only defined in a requirements.txt in the "docs" folder.

### Expected behavior

FreeCAD should not try to install py packages which are not declared in the `package.xml`.

### Actual behavior

FC evaluates the `requirements.txt` in an unrelated folder

### Development version About Info (in Safe Mode)

```shell
OS: macOS 15.7
Architecture: arm64
Version: 26.3.0dev.20260812 (Git shallow)
Build date: 2026/08/12 05:41:29
Build type: Release
Branch: (HEAD detached at dc1ef6e88)
Hash: dc1ef6e880d6f5cf5d5a6eb3749373141eb0deb8
Python 3.11.14, Qt 6.8.3, Coin 4.0.10 (bundled), Pivy 0.6.11 (bundled), Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, Clipper2 , IfcOpenShell 0.8.2, OCC 7.8.1
Locale: English/Germany (en_DE)
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Dark/
QPA/File dialog/Color dialog: cocoa/via Qt/via Qt
Navigation Style/Orbit Style/Rotation Mode: Touchpad/Rounded Arcball/Window center
Logical DPI/Physical DPI/Pixel Ratio: 72/128.5/2
OpenGL version/vendor/renderer: 2.1 Metal - 89.4/Apple/Apple M4
```

### Last known good version (optional)

```shell

```

Reproduce the warning with the FreeCAD-Parametric-Export revision linked in the issue, then trace how the Addon Manager discovers requirements.txt files and reads package.xml declarations. Done means requirements.txt files under unrelated folders such as docs are ignored unless the dependency is declared in package.xml, with coverage for this example behavior.

Written by the indexing model from the issue text.