[cardano-testnet] Extract cardano-testnet-paths micro-package

[palas]

Description

Extracts shared path conventions from cardano-testnet into a new lightweight cardano-testnet-paths micro-package. This enables other packages (e.g. tx-generator) to depend on the same path definitions without pulling in the full cardano-testnet dependency tree.

This refactoring is necessary to enable the realisation of #6511 in a way that is robust (statically checked). Check #6510 for more details.

This PR is meant to be a pure refactoring — there should be no behaviour changes.

Closes #6512 (part 1 of #6510).

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Running tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff

[palas]

Closed in favour of: #6518