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_mir_dataflow/messages.ftl

@@ -0,0 +1,29 @@
+mir_dataflow_path_must_end_in_filename =
+    path must end in a filename
+
+mir_dataflow_unknown_formatter =
+    unknown formatter
+
+mir_dataflow_duplicate_values_for =
+    duplicate values for `{$name}`
+
+mir_dataflow_requires_an_argument =
+    `{$name}` requires an argument
+
+mir_dataflow_stop_after_dataflow_ended_compilation =
+    stop_after_dataflow ended compilation
+
+mir_dataflow_peek_must_be_place_or_ref_place =
+    rustc_peek: argument expression must be either `place` or `&place`
+
+mir_dataflow_peek_must_be_not_temporary =
+    dataflow::sanity_check cannot feed a non-temp to rustc_peek
+
+mir_dataflow_peek_bit_not_set =
+    rustc_peek: bit not set
+
+mir_dataflow_peek_argument_not_a_local =
+    rustc_peek: argument was not a local
+
+mir_dataflow_peek_argument_untracked =
+    rustc_peek: argument untracked