errors: implement sysroot/testing bundle loading

Extend loading of Fluent bundles so that bundles can be loaded from the
sysroot based on the language requested by the user, or using a nightly
flag.

Sysroot bundles are loaded from `$sysroot/share/locale/$locale/*.ftl`.

Signed-off-by: David Wood <david.wood@huawei.com>

Cargo.lock

@@ -1238,16 +1238,6 @@ dependencies = [
  "miniz_oxide",
 ]
 
-[[package]]
-name = "fluent"
-version = "0.16.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "61f69378194459db76abd2ce3952b790db103ceb003008d3d50d97c41ff847a7"
-dependencies = [
- "fluent-bundle",
- "unic-langid",
-]
-
 [[package]]
 name = "fluent-bundle"
 version = "0.15.2"
@@ -3719,7 +3709,8 @@ version = "0.0.0"
 name = "rustc_error_messages"
 version = "0.0.0"
 dependencies = [
- "fluent",
+ "fluent-bundle",
+ "fluent-syntax",
  "rustc_data_structures",
  "rustc_macros",
  "rustc_serialize",