[Feature] Add a private package definition

[ChenyuLInx]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

dbt-cloud is adding support to allow users to define private packages in their dependencies.yml or packages.yml. The format will be similar to git package definition except that the git field will be the private field and an additional optional provider field. Example:

packages:
  - private: dbt-labs/my-awesome-repo
    # optional configs - same as for 'git' packages
    revision: ... # tag, release, branch, or commit SHA
    subdirectory: ...
	provider: github (if there are multiple providers)

In dbt-core, when such a package is defined, we should provide clear error message and tell user to use git definition.

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response