Use newest plugin when multiple versions are found

[yamamoto-febc]

This PR changes to always use newest plugin version when multiple versions found.

background

If you have multiple plugin versions as follows:

$ ls -1 ~/.terraform.d/plugins/
terraform-provider-foobar_v1.0.0_x4
terraform-provider-foobar_v1.1.0_x4

tfschema uses foobar's v1.0.0 or v1.1.0 randomly.

The cause is in the following part.

tfschema/tfschema/client.go

Lines 66 to 72 in 0980d8e

	pluginMetaSet := discovery.FindPlugins(pluginType, dirs)
	for plugin := range pluginMetaSet {
	if plugin.Name == pluginName {
	return &plugin, nil
	}
	}

discovery.FindPlugins returns discovery.PluginMetaSet.
It is defined as map[PluginMeta]struct{}.
https://github.com/hashicorp/terraform/blob/v0.11.7/plugin/discovery/meta_set.go#L7
So its iteration order is not guaranteed.

[minamijoyo]

LGTM 👍

[minamijoyo]

@yamamoto-febc Thank you for your contribution!