Prepare for npm change to _ fields

[vweevers]

npm will soon (TBD) no longer store _ fields in package.json files in node_modules.

This is a problem for us:

prebuild-install/bin.js

Lines 53 to 59 in 652f169

	} else if (!(typeof pkg._from === 'string')) {
	log.info('install', 'installing standalone, skipping download.')
	process.exit(1)
	} else if (pkg._from.length > 4 && pkg._from.substr(0, 4) === 'git+') {
	log.info('install', 'installing from git repository, skipping download.')
	process.exit(1)
	} else if (opts.compile === true || opts.prebuild === false) {

[vweevers]

@ralphtheninja @mafintosh @lovell Do you recall any other places where we rely on _ fields?

[lovell]

Oh, that's a pretty big change. I guess we'll need to check for and read the proposed node_modules/.arborist-resolutions.json file instead first, but the RFC is unclear about what to do if this file is expected but not present; I've added a comment as such.

[vweevers]

I'm kinda hoping there'll be some NPM_* environment variable(s) we can check instead.

[vweevers]

See npm/rfcs#51

[vweevers]

Update: npm/rfcs#51 is superseded by npm/rfcs#90, which introduces additional breaking changes.

[isaacs]

Just for posterity, I want to mention, the shape and filename of the fallback lockfile is still not finalized, and should really be considered an arborist-internal API surface. (Ie, even the npm cli itself won't touch it or depend on it.)

The best approach, I think, is to ensure that scripts have what they need in the environment, so that's what we're trying to maintain in npm/rfcs#90 and friends.