Allow prerelease versions when checking peer deps

[ide]

Summary
Uses lower-level APIs in node-semver to allow prerelease versions when checking peer deps. This honors the intent of library authors e.g. when they write a React component that asks for "react": ">=15.0.0" they're saying OK to react@16.0.0 (partly because the React team specifically has said that if you don't have any warnings in version N, then version N+1 should work for you). By extension and modulo bugs, react@16.0.0-alpha works too if the user explicitly installs it.

Fixes #2760

Test plan
Added unit tests for the new semver utility.

Tested in a real project that uses React 16.0.0-alpha.11 and verified I didn't get any peer dep warnings that usually appear from libraries that ask for "react": ">=15.0.0". Downgraded React to 0.14.0 (too low) and saw peer dep warnings come back as expected.

[bestander]

Thanks, @ide.
Would you fix the lint error please?

[SimenB]

Can this be made to work with engine as well? Haven't actually tested with a pre-release of yarn, but from what I can tell it's separate logic.

yarn/src/package-compatibility.js

Lines 58 to 88 in fe34bb0

	export function testEngine(name: string, range: string, versions: Versions, looseSemver: boolean): boolean {
	const actual = versions[name];
	if (!actual) {
	return false;
	}
	if (!semver.valid(actual, looseSemver)) {
	return false;
	}
	if (semver.satisfies(actual, range, looseSemver)) {
	return true;
	}
	if (name === 'node' && semver.gt(actual, '1.0.0', looseSemver)) {
	// WARNING: this is a massive hack and is super gross but necessary for compatibility
	// some modules have the `engines.node` field set to a caret version below semver major v1
	// eg. ^0.12.0. this is problematic as we enforce engines checks and node is now on version >=1
	// to allow this pattern we transform the node version to fake ones in the minor range 10-13
	const major = semver.major(actual, looseSemver);
	const fakes = [`0.10.${major}`, `0.11.${major}`, `0.12.${major}`, `0.13.${major}`];
	for (const actualFake of fakes) {
	if (semver.satisfies(actualFake, range, looseSemver)) {
	return true;
	}
	}
	}
	// incompatible version
	return false;
	}

$ node --version
v8.2.0-rc.1
$ yarn
yarn install v0.24.6
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
error ansi-styles@3.0.0: The engine "node" is incompatible with this module. Expected version ">=4".
error Found incompatible module
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.

EDIT: Workaround: YARN_IGNORE_ENGINES=true yarn (it was a lerna repo, so just --ignore-engines didn't work)

[bestander]

@SimenB I think it is a good idea, would you send a PR?