Fall back to default Yarn if project Yarn is not set

[charlespierce]

Closes #436

Info

• Currently, if we have a default yarn set, but the user runs yarn within a project that doesn't have yarn specified, then we throw an error and require the user to run volta pin yarn to continue.
• This is a poor UX, as the user has already run volta install yarn, so we do have a Yarn version available, we are just not using it.
• Additionally, with the command passthrough and other UX improvements, we have taken the approach of setting Volta up to support doing things the "safe" way (e.g. if you use yarn in your project, you should specify it there), but not require that they are done that way.
• To align with that goal, we should fall back to the default yarn, even in a project that doesn't have yarn specified.

Changes

• Added an additional platform::Source: ProjectNodeUserYarn, that represents a merged platform.
• Implemented the fall-back logic in session::current_platform, so that the fallback is available to any tool (as running other tools could result in running yarn, e.g. ember new --yarn).
• Updated the tool implementations to account for the new source.

Tested

• Added new acceptance tests to cover the new behavior.

[chriskrycho]

Is this ready to go? If so, I’ll review it and we can merge it! Sorry for the delay; I missed seeing it was open!

[charlespierce]

Yeah, sorry I think I might have forgotten to mention it was out of draft state. Thanks!

[rwjblue]

FWIW, I think GH needs to get better about notifications here. I don't think they send another notification when going from "Draft" to "Ready to review".

[chriskrycho]

CONFIRM, @rwjblue. I'm reviewing now!

[chriskrycho]

This is a solid fix and I think we should ship this so users stop hitting this issue… but I do have a question about whether this is exactly the right overall approach; it seems to me that between this and some of the stuff I've been doing in the volta list implementation, we might have motivation to do some more substantial restructuring around this idea of the source and its relationship to the platform.

[charlespierce]

Created https://github.com/volta-cli/volta/projects/6#card-23308438 and https://github.com/volta-cli/volta/projects/6#card-23308348 to track possible future refactors brought up in PR Review.