Recover app bootstrapping in Python entrypoints

[helderco]

Follow-up to #5060

Python had initial support for entrypoints but because of publishing to Conda, since we didn't publish the extensions code, it was easier to just remove the entrypoint executable from the SDK (in #4399, sdk/python/pyproject.toml):

- [tool.poetry.scripts]
- # FIXME: uncomment when extensions become available
- # dagger-server-py = "dagger.server.cli:app"
- dagger-py = "dagger.cli:app"

This PR recovers __main__.py to enable calling with python -m, without creating the dagger-server-py entrypoint. That executable did allow replacing the entrypoint shell wrapper in the runtime image, apart from needing cd to change the workdir when running the runtime entrypoint. But it also installed it in users venvs, that's why I've not recovered it.

I also simplified installing via requirements.txt. For now we can require it.

In the demo, I've changed the dependency to install SDK library from local dir. It'll require a release for dagger-io[sever] to have the __main__.py module, but it'll also allow iterating on the Python DX going forward.

[helderco]

Fyi: This module is used for codegen:

dagger/sdk/python/pyproject.toml

Lines 128 to 129 in 0c1d7fc

	"python -m dagger generate --output ${GEN_PATH}/gen.py",
	"python -m dagger generate --output ${GEN_PATH}/gen_sync.py --sync",

[helderco]

Fyi: the dagger-server-py name here isn't important. It's just to replace the arg[0] for the command when showing help.

[helderco]

This cd is only needed because at runtime the workdir needs to be where ConfigPath is, not where dagger do is run from.

[helderco]

There's many dependency managers in Python. Also in Node.js there's not only npm, there's pnpm, yarn, etc... We either need to add code to support detecting which one the user is using (limiting to a few most popular), or just support one. For now, let's just require requirements.txt here.

[helderco]

The [server] extra is needed to install Strawberry. It's configured here:

dagger/sdk/python/pyproject.toml

Lines 55 to 58 in 0c1d7fc

	strawberry-graphql = {version = ">=0.133.5", optional = true}
	[tool.poetry.extras]
	server = ["strawberry-graphql"]

[gerhard]

LGTM 🐿