formula_desc_cop desclength: make non-strict

Undeclared dependencies in runtime_dependencies

missing_formula: improve migration messaging.

Particularly in the cask case we can do a better job of communicating
how the new formula can be installed.

Fixes #4089.

components_redundancy_cop: make non-strict

urls_cop pypiurls: make non-strict

components_order_cop: make non-strict

Shorten some links in README.md

Support enabling auto-updates for non-GitHub taps

linkage: fix --test exit code.

Ensure that a non-zero exit code is set both for missing random dylibs
and random missing dependencies.

Additionally, while we are here, drastically trim down the public
interface for this class to the bare minimum and allow getting the
output from `display_test_output` as a variable.

Fixes issue mentioned by @ilovezfs in:
https://github.com/Homebrew/brew/pull/3940#issuecomment-383794520

Fix `MissingFormula` namespacing.

Refactor and fix style for some specs.

x11_requirement: move download to base class.

formula: sort aliases in to_hash.

No reason to have it in macOS class and keeps consistent output with
`brew info` on Linux.

Otherwise the ordering varies based on the filesystem.