Show if rock is supported on other Lua versions on search fail

Supporting changes:
* Change structure of manifest cache: map from repo_url to tables
  mapping from lua versions to manifests.
* New manif_core cache_manifest and get_cached_manifest functions
  to hide cache implementation.
* lua_version optional argument for functions between
  search.find_suitable_rock and manifest loader.

Main changes:
* Add a helper function supported_lua_versions that checks which
  Lua versions can satisfy a query.
* Use this helper function when a search for a rock failed,
  in search.find_suitable_rock, if constraints can be satisfied
  under a different Lua version mention that in the error message.

Examples of error messages:
* Constraint "sailor": "sailor supports only Lua 5.1 and
  Lua 5.2 but not Lua 5.3."
* Constraint "sailor 0.5": "sailor 0.5 supports only Lua 5.1
  and Lua 5.2 but not Lua 5.3."
* Constraint "sailor >= 0.5": "Matching sailor versions support
  only Lua 5.1 and Lua 5.2 but not Lua 5.3."

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