Follow XDG directories spec

[cd-work]

Previously phylum's installer would put all files into ~/.phylum and
retrieve the configuration file from there. This leads to a rather
cluttered home directory which is why the XDG directories spec is
usually preferred.

The CLI will now check the XDG config directory first
(${XDG_CONFIG_HOME:-${HOME}/.config}/phylum/settings.yaml), then fall
back to the old config path if that isn't present.

The installer will put the completions in the XDG data directory
(${XDG_DATA_HOME:-${HOME}/.local/share}/phylum) and the binary in
${HOME}/.local/bin/.

Since the ${HOME}/.local/bin path is sometimes included in the path
already, without being present in the shell config itself, we check the
current $PATH first to try and detect if we need to add it or not.

Closes #224.

[cd-work]

I'd be curious how macOS users feel about ~/.local/share/phylum/completions and ~/.local/bin/phylum`. For Linux that isn't unexpected but macOS users might be confused?

[cd-work]

I think it would likely make sense to put this PR on ice until we've resolved how we want to make things like completions/path modification optional.

It likely makes sense to follow rustup's example and put everything in a separate file where removal and changes are trivial. So it's probably better to change things once rather than having to figure out how to get to that new format from two possibly different starting points.

[louislang]

I think it would likely make sense to put this PR on ice until we've resolved how we want to make things like completions/path modification optional.

I think this makes sense. Should we mark this as a draft so it doesn't accidentally get merged by someone?

[cd-work]

I think this makes sense. Should we mark this as a draft so it doesn't accidentally get merged by someone?

I don't actually think it's possible to convert existing PRs to drafts? Unless you're suggesting to close and re-open as draft.

[cd-work]

Decided to move things into a separate file in this PR, I think that's the easiest choice to avoid having to move things around multiple times.

[louislang]

I don't actually think it's possible to convert existing PRs to drafts? Unless you're suggesting to close and re-open as draft.

You should be able to, just click the Still in progress? Convert to draft under reviewers in the top right!

[samtay]

Worth mentioning that technically Apple specifies using $HOME/Library/Application Support for config/cache dirs like this. For example if you use the directories crate, you can see in their example that this is what they use for a project dir on Mac.

Personally I think this is unexpected for a CLI tool, so I'd vote to keep it under ~/.config/.

However it does raise the question, what is our story for Windows here? Looking at other apps (e.g. neovim, vs code), it seems many use %APPDATA%, but I can't tell if there's a particular standard. Not a Windows user myself.

[cd-work]

Personally I think this is unexpected for a CLI tool, so I'd vote to keep it under ~/.config/.

Yes I agree, I've considered the dirs crate and decided to go this route instead. I would say for data dirs it might be a bit less clear but for config stuff ~/.config should definitely be the right place.

However it does raise the question, what is our story for Windows here? Looking at other apps (e.g. neovim, vs code), it seems many use %APPDATA%, but I can't tell if there's a particular standard. Not a Windows user myself.

It seems like right now Windows support only exists through Linux shims anyway. But I would recommend %APPDATA% should that ever change.