Run optimized dependency install after build

[FredKSchott]

Resolves #475, based on #525

This PR solves all of the tree-shaking issues we've seen since adding it back into Snowpack, mainly around import {TSType} and some of the more custom compile-to-JS languages like Svelte & Vue where the compiler adds imports that our current source code scanner never sees.

This replaces our current build workflow:

1. Install dependencies into a temporary directory (tree-shaking based off the incomplete source code)
2. Copy temp dependencies directory into your build directory
3. Build your source files, resolve dependency imports, and write to disk

With a new, more explicit workflow:

1. Built your source files (not yet resolving imports or saving to disk)
2. Pass these in-memory built files to our installer for tree-shaking analysis
3. Install dependencies directly into build, with tree-shaking based off of complete built source code
4. Now that we have installed dependencies, resolve your imports.
5. Save files to disk.

As a happy bonus, running install as a part of the build means that its output isn't immediately hidden by the first paint of dashboard. Install logs now show up in the Console portion of the dashboard. As a follow-up, we should make this its own section (▼ Install Optimized Dependencies).

Somewhat related: this feels like the final nail in the coffin for representing "where will we install web_modules?" inside of a "mount" script. Now that we have replaced our old "install-then-copy" with an "install directly into build dir", "mounting" this already-fake directory makes even less sense. I think it's time to treat "web_modules" as the special concept that it is, and give it its own "buildOptions.webModulesPath" configuration.

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[drwpow]

Could this printStats() live in the install runner? Or are there times when we don’t want the install command to write to stdout?

[FredKSchott]

Yup, this is in preparation for some of the work happening in #523, and a time when we'll be able to configure install() to not write to stdout/stderr, if you want to call it programatically.

[drwpow]

This makes sense to me! I had some minor questions / comments but nothing blocking.