Change rbx_reflection_database to support loading from FS

[Dekkonot]

As part of the effort to get the reflection database loadable locally, it's probably a good idea to have a central place that decides locations for the entire ecosystem. This will prevent consumers like Lune and Rojo from having to re-implement this logic.

The obvious choice for this, in my opinion, is rbx_reflection_database. It's a requirement for everything that uses the reflection database already, and its public API can be extended to take things from the file system into account without any issues. I've implemented it and documented it thoroughly in the README for rbx_reflection_database.

However, it also sacrifices the 'purity' of the module because it fundamentally alters its purpose. I'm okay with that, but I understand not everyone will be. Please be honest with your thoughts.

I know for sure this works on Windows and Linux. I'm assuming it will also work on MacOS, but I don't own a machine I can use for testing. I don't know how to test this in CI, so we'll just have to be mindful of changes.

[Corecii]

Some minor comments and future planning concerns

[Dekkonot]

Allowing the database to be replaced once it's initialized is complicated because it involves mutating a static variable, so for the time being I'm going to ignore it. Tools like Rojo and Lune can work out solutions without that.

[kennethloeffler]

Looking good, I left a few comments.

RE testing - it looks like the only code paths not covered right now are here:

rbx-dom/rbx_reflection_database/src/lib.rs

Lines 130 to 141 in d95bc80

	} else {
	// Due to concerns about the local data directory existing
	// on Linux, we use the home directory instead.
	#[cfg(target_os = "linux")]
	let mut home = dirs::home_dir()?;
	#[cfg(not(target_os = "linux"))]
	let mut home = dirs::data_local_dir()?;
	home.push(LOCAL_DIR_NAME);
	home.push("database.msgpack");
	Some(home)
	}

Maybe we could add a test that simply compares the result of this to the expected path for the platform, and if we're very paranoid, add some CI targets for macOS and Windows?

[LouieK22]

We should probably use a more generic top-level folder name (e.g. %localappdata%/rbx_dom on Windows, and ~/.rbx_dom on Mac/Linux) as it would give a better hint as to where these files came from. The reflection cache could be something like rbx_dom/cache/reflection.proto

This crate might come in handy for picking directories: https://crates.io/crates/directories

[Dekkonot]

It's currently placed in %localappdata%/.rbx_reflection on Windows, $HOME/Library/Application Support/.rbxreflection on MacOS, and $HOME/.rbxreflection on Linux. .rbx_dom or something works too. It's not really important.

[kennethloeffler]

Looks good, and will work well for consumers that want to update the reflection database without shipping a new binary.

I think the current data directory choosing scheme works fine, probably don't need to add a library for our uses

Ken has approved