(PUP-2093) Migrate PMT to /v3 API.

Prior to this commit, the PMT managed all communication with the Forge via the
(now legacy) /v1 API. That API has been deprecated (primarily due to scalability
concerns), which is less of an issue since it was never officially a public API.

This commit migrates all communication to the Forge to the new /v3 API, which is
expected to be launched as our official public API "soon". This commit also
integrates a standalone dependency resolver, as the /v3 API contains no implicit
dependency resolution (as the /v1 API did); consequently, large portions of the
PMT code have been changed, and others have become unreachable.

This commit also changes the default host that the PMT communicates with, from
https://forge.puppetlabs.com to https://forgeapi.puppetlabs.com (the natural
URL for the Forge API service). Since the tool now expects to communicate with
a completely different service, it will be unable to communicate with services
that do not implement the same /v3 API.