Puppet 3 API functions should be loaded from the pluginsync location as part of the cache system loader

Puppet Version: 6.3.0
Puppet Server Version: N/A
OS Name/Version: Windows 10 - 1803 Ruby 2.5

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Desired Behavior:
Puppet 3 API functions that are plugin sync'd should be loaded from the cache libdir

Actual Behavior:
Puppet 3 API functions are missing

Previously in PUP-6964 (https://github.com/puppetlabs/puppet/pull/7019) the system loader was modified to to be able to load Puppet 3 API functions;

of note - https://github.com/puppetlabs/puppet/pull/7019/files#diff-83a8daf215ff39828e0e3758c2e5a866R34

See how the system loader has loadables of [:func_4x, :func_3x, :datatype]

However when the system cache loader was introduced in PUP-9035 (https://github.com/puppetlabs/puppet/commit/7404ee14745c78bd4a3173c79cac198809faea74 ) the loadables specified did not include the Puppet 3 API functions.

of note - https://github.com/puppetlabs/puppet/pull/6963/files#diff-83a8daf215ff39828e0e3758c2e5a866R30

See how the cache loader has loadables of [:func_4x, :datatype]

The comment at https://github.com/puppetlabs/puppet/pull/6963/files#diff-83a8daf215ff39828e0e3758c2e5a866R22 says they should be the same but is clear they are not.

Also note that the acceptance tests do not test for this;
https://github.com/puppetlabs/puppet/pull/6963/files#diff-d3791ca879b2c6c0586dd8974a7bec0eR25

That is just a Puppet 4 API location