Add "--only-deps" flag to install command

[dciborow]

tl;dr We added --only-deps to flit install

The final implementation is slightly different than originally proposed. The final implementation enables the following command.
flit install --only-deps

Proposal

This extends the CLI, and provides a command wrapper for an additional public function, install_requirements(). This will skip installing the user's code, but will still install all required dependencies. This is particularly useful for preparing building images or containers by enabling users to pre-install dependencies, without installing the source code directly.

This enables installation of dependencies without the need for separate requirements.txt files

flit install-reqs

The following command can be used instead of pip install -r requirements.test.txt or pip install .[test]

flit install-reqs --deps test

Open Issues
The following issues are present, because they are required for normal package installation, but are unnecessarily when installing the dependencies.

• User is required to have "README.md"
• User is required to have package directory

[dciborow]

I have been working with @brettcannon on a template for Python Projects using pyproject.toml. When converting, I removed my requirements.txt and requirements.test.txt, which would previously be parsed in my setup.py. This leaves me a bit stuck on how to install just my dependencies. Before I went and created some crazy TOML parser, i discovered all the needed code was already located in a public method within 'flit', but had not been exposed.

[dciborow]

@takluyver do you have an feedback for this requested feature?

[svartkanin]

Is anything blocking this from getting included?

[takluyver]

Hi, sorry it's taken me so long to look at this - life has been getting in the way. 🙂

I'm happy with the idea of exposing this, but I think I'd prefer to do so as an option for flit install rather than a separate subcommand. How about --only-deps, to mirror pip's --no-deps option?

Poetry has the same thing as --no-root, which I understand - your local project is the root of the dependency tree graph it's dealing with - but I don't find it a particularly clear name.

[takluyver]

I've gone with --only-deps - thanks!

[dciborow]

changing the title so it accurate reflects the implementation.