Fix pkg name during update by using pkg_name understood by hex

[aboroska]

Before this fix warnings were displayed during package upgrade when rebar3 and hex package names were different. For example "mixer" / "inaka_mixer". Such deps are specified like:

{deps, [
        {mixer, "1.0.1", {pkg, inaka_mixer}}
 ]}.

On branch master rebar3 upgrade gives:

===> Verifying dependencies...
===> Failed to update package from repo hexpm
===> Failed to fetch updates for package mixer from repo hexpm

Note the package name ("mixer") in the warning message.

After the fix there is no warning.