Adjust parameter name for Resolver#install_sources

[straight-shoota]

All implementations of #install_sources have a second parameter called install_path, only the abstract definition in Resolver uses the name install_dir, which results in a warning:

positional parameter 'install_path' corresponds to parameter 'install_dir' of the overridden method Shards::Resolver#install_sources(version : Version, install_dir : String), which has a different name and may affect named argument passing

This patch renames the parameter name in the abstract definition to match the one used in all the implementations.