ewoks install: fresh env by default with the pip-venv package manager

[woutdenolf]

In #65 we changed the graph attributes

"requirements": ["ewokscore==5.1.0", "networkx==3.4.2"]

to

"requirements": {
    "python": {"version": "3.12.11", "implementation": "CPython", "...": "..."},
    "system": {"system": "Linux", "machine": "x86_64", "...": "..."},
    "distributions": [
        {"name": "ewokscore", "version": "5.1.0", "installer": "pip"},
        {"name": "networkx", "version": "3.4.2", "installer": "pip"}
    ],
    "manager": {
        "name": "pip",   # Renamed in this PR to "pip-venv"
        "version": "25.0.1",
        "files": {"requirements.txt": "ewokscore==5.1.0\nnetworkx==3.4.2\n"}
    }
}

The final goal is to support more package managers than pip (uv, poetry, pixi and conda).

This PR is another step:

• Refactor the Pip manager to support environment creation and use it by default (rename it to pip-venv).
• In-place installation is opt-in.
• Remove the non-Pip managers to be handled in follow-up PRs.

See docs on how this PR affects ewoks install and ewoks convert

• doc/howtoguides/requirements.rst
• doc/tutorials/install.rst
• doc/tutorials/install/pip_venv.rst

This was cherry-picked from #330 to make the review easier. The other PR already adds uv, pixi, conda and poetry. This PR already lays the groundwork for those in terms of core, test and doc infrastructure.

FYI python -m ewoks._requirements.__init__ does a pip gathering in your current environment. Might be useful for the reviewing.

[codecov]

Codecov Report

❌ Patch coverage is 81.20805% with 112 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/ewoks/_requirements/utils/detect.py	51.11%	22 Missing ⚠️
src/ewoks/_requirements/__init__.py	75.90%	20 Missing ⚠️
src/ewoks/__main__.py	62.79%	16 Missing ⚠️
src/ewoks/_requirements/utils/base_manager.py	84.52%	13 Missing ⚠️
src/ewoks/bindings.py	43.75%	9 Missing ⚠️
src/ewoks/_requirements/utils/files.py	72.00%	7 Missing ⚠️
src/ewoks/_requirements/utils/requirements_txt.py	93.02%	6 Missing ⚠️
src/ewoks/_requirements/utils/process.py	79.16%	5 Missing ⚠️
...quirements/utils/metadata/from_requirements_txt.py	60.00%	4 Missing ⚠️
.../ewoks/_requirements/utils/metadata/last_resort.py	20.00%	4 Missing ⚠️
... and 3 more

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