Hide dependencies in tools.go from importers

Projects that use `go mod vendor` will vendor `github.com/matryer/moq`
despite it not being required at runtime.

Moving `tools.go` to `internal` hides this import from downstream
users and avoids `github.com/matryer/moq` being vendored.

`go generate` of the mocks still works as expected.

The assumption behind the import test broke, so I've pointed it at a
different path that has no Go code. This seems to match the intent
behind the original test for the `internal/code/..` path.

internal/code/imports_test.go

@@ -40,6 +40,6 @@ func TestNameForDir(t *testing.T) {
 
 	assert.Equal(t, "tmp", NameForDir("/tmp"))
 	assert.Equal(t, "code", NameForDir(wd))
-	assert.Equal(t, "internal", NameForDir(wd+"/.."))
+	assert.Equal(t, "docs", NameForDir(wd+"/../../docs"))
 	assert.Equal(t, "main", NameForDir(wd+"/../.."))
 }