Move install logic into separate package

[FredKSchott]

Resolves #312

Background - Community

• We've seen a few requests come in over the last year for a way to run Snowpack's CJS->ESM installation pipeline in contexts outside of Snowpack the web app builder. Add programmatic install interface #525 contains most of these convos.
• As Node.js ESM begins to enter the mainstream, the node community will begin to need something like Snowpack as well (see https://changelog.com/jsparty/137)
• Some have attempted this in new projects, like https://github.com/mikeal/brrp, which are basically "Snowpack for Node"

Background - Snowpack

• Since Snowpack v2.0, we've been trending more towards "Snowpack the build tool" and away from "Snowpack the package installer". Installing packages is still a hugely important feature that all of Snowpack is based on, but the idea of Snowpack trying to be both tools at the same time has been weighing on us (and makes Snowpack harder to communicate, document, etc).
• As a result, we've tended to prioritize the build tool and failed to support the installer as well as we should, leading to a worse, sometimes-confusing experience for everyone.
• We have a larger initiative that we'd like to kick off that will require this separation between Snowpack the build tool and Snowpack the package installer. (More on this later :)

New Package: esmpkg

Today, this is just the idea of moving Snowpack's "install" command into a new package. This means no change to how Snowpack works, but a few benefits right off the bat:

• Our installer finally gets an official, JS interface that people have been asking about for a while.
• Easier to document esmpkg & Snowpack individually vs. overloading the Snowpack docs.

In the future, esmpkg can lazer-focus on improvements to support better CJS->ESM conversions and begin working on conversions for Node.js (in addition to our current web support).

Testing

• Covered by existing tests.

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[drwpow]

Testing: Still TODO

I’d be curious how much effort it’d be to just rename tests/integration -> tests/esmpkg and make updates there. Since this has a Node interface, we’d probably be able to test the output even easier? Either way, it feels like those install tests should move away from Snowpack and into esmpkg. Should be a great start for esmpkg, and would simplify Snowpack’s tests too.

[FredKSchott]

+1, but how do you feel about doing that in a follow up PR? I'm worried it will explode the PR making it harder to review yolo, did it here.

[FredKSchott]

@drwpow @gr2m PTAL

[gr2m]

I just looked through the code changes, I didn't test the changes in detail. Let me know if you want me to have a thorough look

[gr2m]

that doesn't look right?

[drwpow]

I think this makes sense; it’s a hook that happens before publishing to npm making sure a fresh build is prepared (i.e. it’s not stale, from a previous commit or different branch)

[gr2m]

the "version" script is a hook that happens before publishing? Are you sure?

[drwpow]

Not technically it doesn’t, but in our workflow since we use Lerna, the versioning happens before publishing (I think? Please correct me if I’m still mistaken!).

[drwpow]

Looks good! I didn’t compare how much esmpkg deviated from the original install script, but nothing seemed to jump out. Seems straightforward, and the updated tests give me confidence it’s working the same. So excited for this!!! 💯 💯