initwd: NewModuleInstaller takes package fetcher as dependency arg

Earlier work started to reshape this API to follow the dependency inversion
style, but didn't go so far as treating the package fetcher as an argument
because so far it hasn't offered any customizable policy anyway.

In future commits we will be introducing some policy arguments for the
package fetcher, and so this is some preparation work where we move the
responsibility for calling getmodules.NewPackageFetcher() out into the
caller of initwd.NewModuleInstaller().

This changes the API consumed by a bunch of unit testing helpers, so
splitting this out into its own commit should hopefully make future
commits more focused. The module installer now explicitly supports being
instantiated without a registry client or a remote package fetcher and
will in that case return an error if it's asked to install from anywhere
other than local relative directories. Most of our existing tests are
comfortable running under that constraint and so will not need any further
work in later commits that will change the signature of
getmodules.NewPackageFetcher.

However, a couple tests in package initwd _itself_ were making use of the
esoteric legacy support for treating an absolute filesystem path as a funny
sort of remote source, and so for now those will instantiate their own
package fetcher. Future commits that change the NewPackageFetcher signature
will need to offer a concession for those two tests.

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

internal/initwd/from_module.go

@@ -96,7 +96,7 @@ func DirFromModule(ctx context.Context, loader *configload.Loader, rootDir, modu
 	}
 
 	instDir := filepath.Join(rootDir, ".terraform/init-from-module")
-	inst := NewModuleInstaller(instDir, loader, reg)
+	inst := NewModuleInstaller(instDir, loader, reg, getmodules.NewPackageFetcher())
 	log.Printf("[DEBUG] installing modules in %s to initialize working directory from %q", instDir, sourceAddrStr)
 	os.RemoveAll(instDir) // if this fails then we'll fail on MkdirAll below too
 	err := os.MkdirAll(instDir, os.ModePerm)