deps: upgrade npm to 2.8.3

[othiym23]

The npm team has been very busy over the last week, partially working on getting issues related to scoped modules straightened out prior to the launch of private packages on the npm, Inc. registry, but mostly to get git working as robustly as possible with npm, so I could stop thinking about it for a while. Because there were many intermediate releases as we fixed bugs, the combined changelog is massive, but I've worked hard to make the release notes readable, so:

• https://github.com/npm/npm/releases/tag/v2.8.0
• https://github.com/npm/npm/releases/tag/v2.8.1
• https://github.com/npm/npm/releases/tag/v2.8.2
• https://github.com/npm/npm/releases/tag/v2.8.3

The reason for the semver-minor version bump, taken from the 2.8.0 release notes, is:

• EXPERIMENTAL FEATURE: Add support for github:, gist:, bitbucket:,
  and gitlab: shorthand prefixes. GitHub shortcuts will continue to be
  normalized to org/repo instead of being saved as github:org/repo, but
  gitlab:, gist:, and bitbucket: prefixes will be used on the command
  line and from package.json. BE CAREFUL WITH THIS. package.json files
  published with the new shorthand syntax can only be read by [email protected] and
  later, and this feature is mostly meant for playing around with it. If you
  want to save git dependencies in a form that older versions of npm can read,
  use --save-exact, which will save the git URL and resolved commit hash of
  the head of the branch in a manner simiilar to the way that --save-exact
  pins versions for registry dependencies. This is documented (so check npm help install for details), but we're not going to make a lot of noise about
  it until it has a chance to bake in a little more.

A large portion of the diff is changes to the tests that I made in an effort to clean them up and to make it easier for contributors to write new tests without getting confused about how they should look.

This supersedes #14421 and #14592.

[misterdjules]

Superseded by #20311. Thank you once again @othiym23.