Indexes 8: Fixes the spk build or mkb crash with indexes enabled

[dcookspi]

This fixes a crash that happens when using spk build or spk mkb with indexes enabled. spk would panic because downstream_build_requirements() was not implemented for IndexedPackage objects. Implementing this method using the existing index data fixed the problem.

This also refactors the existing downstream_runtime_requirements() method to use a filter function based shared downstream_requirements() method - along the same lines as the exisitng v0/PackageSpec does. Both downstream_*_requirements methods are added to the flatbuffer index tests.

This is the 8th of the chained PRs for adding indexes to spk solves:

1. Indexes 1: Change Package and related traits to not return references to fields #1336
2. Indexes 2: Add new_unchecked() constructors to spk schema objects #1337
3. Indexes 3: Adds flatbuffers schema and SolverPackageSpec for indexes to spk #1338
4. Indexes 4: Adds Indexes for SPK repositories #1339
5. Indexes 5: Adds spk repo index subcommand for index generation and updates #1340
6. Indexes 6: Changes version_filter field in index schema #1344
7. Indexes 7: Adds a lock file around index generation and updates #1354
8. this PR
9. Indexes 9 - Adds messaging on package events to kafka #1356

[codecov]

Codecov Report

❌ Patch coverage is 68.29268% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/spk-schema/src/v0/indexed_package.rs	62.85%	13 Missing ⚠️

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