Implement v2-gen-bounds function

[mpickering]

This commit implements project-aware functionality for the cabal gen-bounds command, allowing it to work correctly in multi-package projects. Previously, running gen-bounds from within a package directory that depends on another local package would fail because it couldn't find the local dependency.

The implementation follows the same pattern as other v2 commands, creating a full project context that knows about all packages defined in the cabal.project file. This allows gen-bounds to properly analyze dependencies between local packages and suggest appropriate bounds.

cabal gen-bounds <TARGET>

Fixes #7504
Fixes #8654
Fixes #9752
Fixes #5932

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