Add project_id column to packages_dependencies table

What does this MR do and why?

The packages_dependencies table currently is shared for all packages. That needs to be changed in order to have a project_id sharding key.

This MR adds project_id column to packages_dependencies table and changes the uniqueness validation and UNIQUE database indexes to consider project_id column:

1. Deletes the existing UNIQUE index on name, version_pattern.
2. Adds the temporary UNIQUE index on name, version_pattern WHERE project_id IS NULL.
3. Adds the new UNIQUE index on name, version_pattern, project_id WHERE project_id IS NOT NULL;

The project_id backfilling and setting it for new records will be added in the separate MRs.

MR acceptance checklist

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How to set up and validate locally

Run the migrations rails db:migrate:main

Related to #465276 (closed)