Cleanup and refactoring

[nicoddemus]

Doing some cleanup preparing for the work on lock files:

• Move to src layout.
• Drop support for Python 3.7, 3.8 and 3.9: 3.7 will be EOL in two weeks, however makes sense to drop support for older versions because one can still use a Python 3.10+ root to manage sub-environments in those older versions.
• Call conda and mamba using subprocess: this simplifies the code and also allow us to use the conda-devenv script from anywhere, without needing for it to be installed in the root. This facilitates development/testing.
• Other small improvements

EDEN-2633

[nicoddemus]

One question: does it have integration tests that call conda or mamba directly without mocking?

Currently only via mocking. :/