Move Gazelle extension to //gazelle/bzl and change package name

This fixes an issue with importing bazel-skylib into
google3. Currently, Glaze (internal Go build file generator) attempts
to generate a target (//gazelle:gazelle) that conflicts with one
that's already declared here.

I think the right solution is actually to move the package into a
subdirectory. In the future (bazel-contrib/bazel-gazelle#5), Gazelle's Go
extension will generate target names similar to what Glaze does, so
the same conflict will happen in open source. I think it's also
logical to have a directory of packages in case more need to be added
in the future, and for the extension to have a package name matching
the language it works with.

This is an incompatible change, but the //gazelle directory isn't part
of a tagged release yet, so hopefully it won't break anyone.

gazelle/gazelle.go → gazelle/bzl/gazelle.go

@@ -21,7 +21,7 @@ limitations under the License.
 //
 // This extension is experimental and subject to change. It is not included
 // in the default Gazelle binary.
-package gazelle
+package bzl
 
 import (
 	"flag"

gazelle/gazelle_test.go → gazelle/bzl/gazelle_test.go

@@ -1,4 +1,4 @@
-package gazelle
+package bzl
 
 /* Copyright 2020 The Bazel Authors. All rights reserved.