Packages: Add min stability and prefer-stable, and fix dep tree

[tyxla]

After the latest commits in feature/jetpack-packages, it seems clean installation of composer inside packages doesn't work, because of version constraints:

Changes proposed in this Pull Request:

• Add min stability and prefer-stable to all packages and the root one, and fix dependency tree

Is this a new feature or does it add/remove features to an existing part of Jetpack?

Part of the Jetpack DNA project - p1HpG7-70O-p2

Testing instructions:

• Go to the root directory of the repo.
• rm -rf ./vendor
• rm -rf ./packages/**/vendor
• rm ./packages/**/composer.lock
• Try composer install and verify it works.
• Try composer install in each package and verify it works.

Note: tests are expected to fail - this was introduced in #12562

Proposed changelog entry for your changes:

• Add min stability and prefer-stable to all packages and the root one, and fix dependency tree

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: July 2, 2019.
Scheduled code freeze: June 25, 2019

Generated by 🚫 dangerJS against d4fd39f

[tyxla]

Also, it's interesting that with #12571 that reverts #12562, tests are passing and there are no more dependency problems.

[tyxla]

Tests are passing after the revert (https://travis-ci.org/Automattic/jetpack/builds/541633590) so I'm going to close this one for now as it's unnecessary.