* Fix to show proper error message when git is not installed

* Added test case for when git is not installed

Fixes #2204

The phrase "doesn't depend on" can be interpreted as "doesn't use" and
considered an unnecessary import. Rephrase to be more clear that the
message is discussing the dependencies in pubspec.yaml.

This documentation relates to when Dart 1.x when pub did more than
fetching dependencies and pre-compiling executables.

* return the hashCode of the description

* use _parseDescription(description).hashCode as the hashDescription

* Cover common use-cases with caret notation.
 * Test that dependency on pre-release from stable release causes usage
   of the pre-release.
 * Test that direct dependency on stable release will not be overwritten
   by pre-release dependency from other stable dependency.
 * Test limitations of backtracking for dependency on pre-releases.

Wrong number in graph fixed

* Purge invalid git repository from cache before cloning.

* Test cases for invalid git repositories

The `analysis_options.yaml` is not part of the package.
It's for the editor, it should not affect which binaries are loaded
or how dependencies are analyzer when publishing.

Also remove codereview.settings

Don't validate imports in `example/`.

Change `flutter packages` to `flutter pub`

This ensures that we always have an email and a user_id / id_token
on the server.

- Point users to the correct repo for problems with `pub.dev`.
- Add sections to fill out with enough info to debug.
- Encourage detailed features requests.

This was incorrectly removed in #1972. In the case of the `lish` command
the default was unused, but the `uplaoders` command still used it.

Fixes #2024

There is nothing that requires a library name to be a valid Dart
identifier, and it's common practices to at least have `part of` files
which include a `.` in the file name.

Fixes https://github.com/dart-lang/pub/issues/2066