Previously we had a "getProvider" function type used to implement plugin
fetching. Here we replace that with an interface type, initially with
just a "Get" function.
For now this just simplifies the interface by allowing the target
directory and protocol version to be members of the struct rather than
passed as arguments.
A later change will extend this interface to also include a method to
purge unused plugins, so that upgrading frequently doesn't leave behind
a trail of unused executable files.
When running "terraform init" with providers that are unconstrained, we
will now produce information to help the user update configuration to
constrain for the particular providers that were chosen, to prevent
inadvertently drifting onto a newer major release that might contain
breaking changes.
A ~> constraint is used here because pinning to a single specific version
is expected to create dependency hell when using child modules. By using
this constraint mode, which allows minor version upgrades, we avoid the
need for users to constantly adjust version constraints across many
modules, but make major version upgrades still be opt-in.
Any constraint at all in the configuration will prevent the display of
these suggestions, so users are free to use stronger or weaker constraints
if desired, ignoring the recommendation.
As well as constraining plugins by version number, we also want to be
able to pin plugins to use specific executables so that we can detect
drift in available plugins between commands.
This commit allows such requirements to be specified, but doesn't yet
specify any such requirements, nor validate them.
The semver library we were using doesn't have support for a "pessimistic
constraint" where e.g. the user wants to accept only minor or patch
version upgrades. This is important for providers since users will
generally want to pin their dependencies to not inadvertantly accept
breaking changes.
So here we switch to hashicorp's home-grown go-version library, which
has the ~> constraint operator for this sort of constraint.
Given how much the old version object was already intruding into the
interface and creating dependency noise in callers, this also now wraps
the "raw" go-version objects in package-local structs, thus keeping the
details encapsulated and allowing callers to deal just with this package's
own types.
