Simplify message paths

This makes it easier to open the messages file while developing on features.

The commit was the result of automatted changes:

for p in compiler/rustc_*; do mv $p/locales/en-US.ftl $p/messages.ftl; rmdir $p/locales; done

for p in compiler/rustc_*; do sed -i "s#\.\./locales/en-US.ftl#../messages.ftl#" $p/src/lib.rs; done

compiler/rustc_middle/messages.ftl

@@ -0,0 +1,36 @@
+middle_drop_check_overflow =
+    overflow while adding drop-check rules for {$ty}
+    .note = overflowed on {$overflow_ty}
+
+middle_opaque_hidden_type_mismatch =
+    concrete type differs from previous defining opaque type use
+    .label = expected `{$self_ty}`, got `{$other_ty}`
+
+middle_conflict_types =
+    this expression supplies two conflicting concrete types for the same opaque type
+
+middle_previous_use_here =
+    previous use here
+
+middle_limit_invalid =
+    `limit` must be a non-negative integer
+    .label = {$error_str}
+
+middle_const_eval_non_int =
+    constant evaluation of enum discriminant resulted in non-integer
+
+middle_unknown_layout =
+    the type `{$ty}` has an unknown layout
+
+middle_values_too_big =
+    values of the type `{$ty}` are too big for the current architecture
+
+middle_cannot_be_normalized =
+    unable to determine layout for `{$ty}` because `{$failure_ty}` cannot be normalized
+
+middle_strict_coherence_needs_negative_coherence =
+    to use `strict_coherence` on this trait, the `with_negative_coherence` feature must be enabled
+    .label = due to this attribute
+
+middle_const_not_used_in_type_alias =
+    const parameter `{$ct}` is part of concrete type but not used in parameter list for the `impl Trait` type alias