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_interface/messages.ftl

@@ -0,0 +1,52 @@
+interface_ferris_identifier =
+    Ferris cannot be used as an identifier
+    .suggestion = try using their name instead
+
+interface_emoji_identifier =
+    identifiers cannot contain emoji: `{$ident}`
+
+interface_mixed_bin_crate =
+    cannot mix `bin` crate type with others
+
+interface_mixed_proc_macro_crate =
+    cannot mix `proc-macro` crate type with others
+
+interface_error_writing_dependencies =
+    error writing dependencies to `{$path}`: {$error}
+
+interface_input_file_would_be_overwritten =
+    the input file "{$path}" would be overwritten by the generated executable
+
+interface_generated_file_conflicts_with_directory =
+    the generated executable for the input file "{$input_path}" conflicts with the existing directory "{$dir_path}"
+
+interface_temps_dir_error =
+    failed to find or create the directory specified by `--temps-dir`
+
+interface_out_dir_error =
+    failed to find or create the directory specified by `--out-dir`
+
+interface_cant_emit_mir =
+    could not emit MIR: {$error}
+
+interface_rustc_error_fatal =
+    fatal error triggered by #[rustc_error]
+
+interface_rustc_error_unexpected_annotation =
+    unexpected annotation used with `#[rustc_error(...)]`!
+
+interface_failed_writing_file =
+    failed to write file {$path}: {$error}"
+
+interface_proc_macro_crate_panic_abort =
+    building proc macro crate with `panic=abort` may crash the compiler should the proc-macro panic
+
+interface_unsupported_crate_type_for_target =
+    dropping unsupported crate type `{$crate_type}` for target `{$target_triple}`
+
+interface_multiple_output_types_adaption =
+    due to multiple output types requested, the explicitly specified output file name will be adapted for each output type
+
+interface_ignoring_extra_filename = ignoring -C extra-filename flag due to -o flag
+
+interface_ignoring_out_dir = ignoring --out-dir flag due to -o flag