maint: Move Guix files to '.guix'.

* build-aux/guix: Rename to...
* .guix/modules: ... this.
* .guix-channel: Adjust accordingly.
* guix.scm: Likewise.
* build-aux/manifest.scm: Rename to...
* .guix/manifest.scm: ... this.
* Makefile.am (EXTRA_DIST): Adjust accordingly.

.guix-channel

@@ -2,4 +2,4 @@
 
 (channel
   (version 0)
-  (directory "build-aux/guix"))
+  (directory ".guix/modules"))

Makefile.am

@@ -83,8 +83,8 @@ EXTRA_DIST = LICENSE HACKING GUILE-VERSION			\
 	     libguile/texi-fragments-to-docstrings		\
 	     gdbinit						\
 	     guix.scm						\
-	     build-aux/guix/guile-package.scm			\
-	     build-aux/manifest.scm
+	     .guix/modules/guile-package.scm			\
+	     .guix/manifest.scm
 
 TESTS = check-guile
 TESTS_ENVIRONMENT = @LOCALCHARSET_TESTS_ENVIRONMENT@

guix.scm

@@ -1 +1 @@
-build-aux/guix/guile-package.scm
+.guix/modules/guile-package.scm