This flattens the getter package tree, adds tests, and changes a little
bit of the terminology to follow Go idioms. This also makes much of the
getter API private to begin with. This will give us more flexibility in
the future.
It is now possible to create plugins with chart download capabilities for custom, non-http protocols.
Furthermore it is possible to reuse helm packages to implement alternative clients with these custom downloader functions.
When downloader package moved to under /pkg, helmpath functions got called still from it, while it is under /cmd. This commit fixes the issue by moving helmpath after the downloader to have only cmd->pkg and pkg->pkg calls.