Mxdev

- Removed leftover print [jensens]

- Fix usage of `--install-option='pre'` and use `--pre` option in requirements files instead.
The install options are deprecated in pip 23 which Plone switched to recently.
More info:
https://github.com/pypa/pip/issues/11358
https://discuss.python.org/t/passing-command-line-arguments-to-pip-install-after-install-options-deprecation/22981/6
[thet, fredvd]

- Fix reading sections from the config parser without defaults if the section contains a setting that also exists as default.
[rnix]

- Do not write constraints out to the file if no constraints are defined.
[rnix]

- Add the `main-package` option to the settings.
[rnix]

- In this package, use `pyproject.toml` and markdown for README et al.
[jensens]

- Add `use` option to sources, and `default-use` to the settings.
`default-use` is true by default.
When false, the source is not checked out, and the version for this package is not overridden.
[maurits]

- Do not use `libvcs`, but recycled and updated (type hints, tests) `mr.developer` VCS code.
Code for GIT is tested well, code for SVN, Mercurial, Bazaar and DARCS needs contributors with knowledge in this area.
Additional options, like `pushurl`, ... (see README) were added.
`pip` style VCS URLs are not supported any longer.
[jensens, rnix, zworkb]

- Config parser options are now considered case-sensitive.
[rnix]

- Do not fail `mxdev` run if `requirements.txt` is missing.
[rnix]

- Add flag to only fetch repositories and skip generating files.
[rnix]

- Add flag to skip fetching of repositories.
[rnix]

- Add support for custom hooks.
[rnix]

- Rename `sources.ini` to `mx.ini` in the documentation.
[rnix]

- Introduce state object and pass it to read/fetch/write.
State object contains all required runtime data.
[rnix]

- Depend on pip 22, where interdependency mode is no longer needed.
Remove all interdependency-related code.
[jensens]

- Better error message if the requirements-in file does not exist.
[jensens]

- Better last message with the full pip command.
[jensens]

- Allow empty `requirements-in` configuration.
[jensens]

- Feature: Ignore existing constraints.
New setting `ignores` with a list of packages (one per line) to ignore constraints without providing a version.
[jensens]