getmodules: NewPackageFetcher now expects an "environment" argument

This continues our work to follow the dependency inversion style for the
"package fetcher" component of the module installer.

Mimicking the existing pattern for providers, package main is now
responsible for instantiating the PackageFetcher and providing it to
the "command" package as a field of command.Meta.

We could potentially go further here and follow dependency inversion style
for _all_ of the special clients needed by the various go-getter getters,
but our primary concern for now is preparing to add a new "getter" for
installation from an OCI Distribution repository, and so we'll leave the
other already-working code unchanged to reduce the risk of this initial
work.

Future commits will actually wire in the implementation details for OCI
Repository access. This commit focuses only on plumbing the necessary
objects through the API layers.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

internal/command/get_test.go

@@ -10,6 +10,8 @@ import (
 	"testing"
 
 	"github.com/mitchellh/cli"
+
+	"github.com/opentofu/opentofu/internal/getmodules"
 )
 
 func TestGet(t *testing.T) {
@@ -103,6 +105,14 @@ func TestGet_cancel(t *testing.T) {
 			Ui:               ui,
 			WorkingDir:       wd,
 			ShutdownCh:       shutdownCh,
+
+			// This test needs a real module package fetcher instance because
+			// its configuration includes a reference to a module from a registry
+			// that doesn't really exist. The shutdown signal prevents us from
+			// actually making a request to this, but we still need to provide
+			// the fetcher so that it will _attempt_ to make a network request
+			// that can then fail with a cancellation error.
+			ModulePackageFetcher: getmodules.NewPackageFetcher(nil),
 		},
 	}