Split meshroom.core.desc module into a package with submodules

[yann-lty]

Description

This PR does not bring any functional changes - it's only about code re-organization.
The main goal is to split the current meshroom/core/desc.py file into its own package with submodules, to apply the single-responsibility-principle and break a large module into more granular and manageable code units.

From:

meshroom/core/desc.py

To:

meshroom/core/desc/
    __init__.py
    attribute.py
    computation.py
    node.py

The API of the module has been preserved, therefore the node plugins are not impacted by this internal change.

Features list

• Split meshroom/core/desc.py into a package with submodules while keeping git history.

Implementation remarks

To maintain git history in each new file, the original desc.py file has been git-moved to those new submodule paths on separate branches, which all got merged afterwards (hence the number of commits here).