Implement packages and libraries

[jonmeow]

Tracking issue for implementing the package and import keywords, for packages and libraries.

ref: https://github.com/carbon-language/carbon-lang/tree/trunk/docs/design/code_and_name_organization

[github-actions]

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This issue is labeled inactive because the last activity was over 90 days ago.

[chandlerc]

Closing explorer-specific issues as not-planned for now due to our decision to prioritize working on the toolchain over other implementation work in the near term: https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3532.md