Respect ignore engines flag.

[samccone]

Test plan

package.json

{
  "dependencies": {
  "cosmiconfig": "2.0.1"
  }
}

yarn install

Fails
...

yarn install --ignore-flags

works

Fixes #638

[bestander]

@samccone would you mind fixing lint?

src/package-compatibility.js:101
101:     this.ignoreEngines = ignoreEngines;
         ^^^^^^^^^^^^^^^^^^ assignment of property `ignoreEngines`
101:     this.ignoreEngines = ignoreEngines;
              ^^^^^^^^^^^^^ property `ignoreEngines`. Property not found in
 96: export default class PackageCompatibility {
                          ^^^^^^^^^^^^^^^^^^^^ PackageCompatibility

[samccone]

done @bestander 🐐