[C++] Third party download URLs are duplicated

[asfimport]

Currently, third-party source download URLs are specified in both cpp/thirdparty/versions.txt and cpp/cmake_modules/ThirdPartyToolchain.cmake. It would be nice to find a way to deduplicate the information.

Reporter: Antoine Pitrou / @pitrou

Related issues:

• [C++] Consolidate thirdparty bundle URLs, version bumping logic, etc (duplicates)

_{Note: This issue was originally created as ARROW-6333. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
Transitioning issue from Resolved to Closed to based on resolution field value.