message: Fix visibility to match reality.
  This also lets us remove pack.h and matrix.h from hdrs of :ruy, which is nice.
diff: @@ -163,6 +163,7 @@ cc_library(
  cc_library(
  name = "path",
  hdrs = ["path.h"],
  + visibility = ruy_visibility(),
  deps = [":size_util"],
  )
  @@ -205,12 +206,14 @@ cc_library(
  cc_library(
  name = "matrix",
  hdrs = ["matrix.h"],
  + visibility = ruy_visibility(),
  deps = [":check_macros"],
  )
  cc_library(
  name = "spec",
  hdrs = ["spec.h"],
  + visibility = ruy_visibility(),
  deps = [":matrix"],
  )
  @@ -306,8 +309,6 @@ cc_library(
  "prepack.h",
  ],
  hdrs = [
  - "matrix.h",
  - "path.h",
  "ruy.h",
  "ruy_advanced.h",
  ],
  @@ -316,6 +317,8 @@ cc_library(
  ":check_macros",
  ":common",
  ":context",
  + ":matrix",
  + ":path",
  ":size_util",
  ":spec",
  ":trmul",
  