build.txt (requirements file) is missing exif, lxml and rectangle-packer

[dginovker]

Describe the bug
When trying to fix up the Flatpak build for Beeref, I hit these issues:

Traceback (most recent call last):
  File "beeref/__main__.py", line 30, in <module>
  File "PyInstaller/loader/pyimod02_importers.py", line 419, in exec_module
  File "beeref/view.py", line 28, in <module>
  File "PyInstaller/loader/pyimod02_importers.py", line 419, in exec_module
  File "beeref/fileio/__init__.py", line 22, in <module>
  File "PyInstaller/loader/pyimod02_importers.py", line 419, in exec_module
  File "beeref/fileio/image.py", line 24, in <module>
ModuleNotFoundError: No module named 'exif'
[3] Failed to execute script '__main__' due to unhandled exception!

Traceback (most recent call last):
  File "beeref/__main__.py", line 30, in <module>
  File "PyInstaller/loader/pyimod02_importers.py", line 419, in exec_module
  File "beeref/view.py", line 28, in <module>
  File "PyInstaller/loader/pyimod02_importers.py", line 419, in exec_module
  File "beeref/fileio/__init__.py", line 22, in <module>
  File "PyInstaller/loader/pyimod02_importers.py", line 419, in exec_module
  File "beeref/fileio/image.py", line 25, in <module>
ModuleNotFoundError: No module named 'lxml'
[3] Failed to execute script '__main__' due to unhandled exception!

Traceback (most recent call last):
  File "beeref/__main__.py", line 30, in <module>
  File "PyInstaller/loader/pyimod02_importers.py", line 419, in exec_module
  File "beeref/view.py", line 33, in <module>
  File "PyInstaller/loader/pyimod02_importers.py", line 419, in exec_module
  File "beeref/scene.py", line 23, in <module>
ModuleNotFoundError: No module named 'rpack'
[3] Failed to execute script '__main__' due to unhandled exception!

Both were resolved by adding exif and lxml to build.txt

System info

To Reproduce
Steps to reproduce the behavior:

1. Get a barebones Linux installation
2. Try to compile Beeref

[dginovker]

Btw if you don't think this is an issue, feel free to close

[rbreu]

The requirements for BeeRef itself come from installing the beeref package itself, they are defined in pyproject.toml. So in your git clone directory, run pip install . --upgrade

The build.txt only holds additional requirements needed for the building tools (which is at the moment only pyinstaller). Same for all the other files in the requirements folder.