plan.json: Include compiler-abi field

[mpickering]

The plan.json file was missing crucial information needed to locate
packages in the store directory structure. When cabal-install stores
packages, it uses a full compiler identifier including an ABI tag (e.g.,
"ghc-9.10.1-69c3") but plan.json only included the basic compiler ID
("ghc-9.10.1"). This made it impossible for external tools like
cabal-plan to correctly locate packages and their files in the store.

This commit adds a new "compiler-abi" field with the ABI tag string

The issue reported in the original ticket seems to be about the location
of LICENSE files, but the plan.json may still lack sufficient
information to locate all license files since the packages may not be
located in the store, but in different package databases given by
the --package-db flag.  This will do for now.

Fixes #10726

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