Instead of searching the installed provider package directory for a
binary as we install it, we can lazily detect the executable as it is
required. Doing so allows us to separately report an invalid unpacked
package, giving the user more actionable error messages.
Historically our logic to handle discovering and installing providers has
been spread across several different packages. This package is intended
to become the home of all logic related to what is now called "provider
cache directories", which means directories on local disk where Terraform
caches providers in a form that is ready to run.
That includes both logic related to interrogating items already in a cache
(included in this commit) and logic related to inserting new items into
the cache from upstream provider sources (to follow in later commits).
These new codepaths are focused on providers and do not include other
plugin types (provisioners and credentials helpers), because providers are
the only plugin type that is represented by a heirarchical, decentralized
namespace and the only plugin type that has an auto-installation protocol
defined. The existing codepaths will remain to support the handling of
the other plugin types that require manual installation and that use only
a flat, locally-defined namespace.
