Add universal hosted git URL support

[evocateur]

Description

Improve the separation of concerns between Package instances and their consumers by introducing the updateDependency() instance method. By judiciously employing npm-package-arg and the new PackageGraph metadata, we no longer have to serialize/deserialize the entire object as an extra step during command preparation.

Remaining questions for @gustaff-weldon or @synaptiko:

1. Is the --git-version-prefix flag really necessary? Who actually needs to change that sort of thing?
2. Are there any drawbacks to always recognizing these sorts of purposeful git dependencies (they only ever match local packages)?
3. If git urls are always updated as implicitly --exact, do we need to keep throwing an error if that option is missing?

Breaking Changes

• VersionSerializer and GitVersionParser have been removed

  We now simply recognize any local package name with a matching git url, prefix or not. npm-package-arg is pretty handy for this. The documentation still needs updating.

Motivation and Context

Having PublishCommand mutating internal properties of Package instances by reference always bugged me. Now it's better encapsulated. This will make future refactoring easier.

How Has This Been Tested?

Added fixture-based tests of the git-url support added to lerna publish in #1033! Also, a few more pieces of #1262 validation.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[lock]

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