Fix file name when using the import_fetcher option

When using the import_fetcher option, the file name of
fetched content was accidentally set to "from_fetched.proto".

This could cause problems when there were more than one import
statement, because gpb would consider all to originate from the
file "from_fetched.proto", and this would result in an error:

  {multiply_defined_file_or_files,["from_fetched.proto"]}

Fix by using the file name in the import statement.

src/gpb_compile.erl

@@ -3841,8 +3841,8 @@ locate_read_input_once(Input, CheckSeen, ImEnv) ->
             {error, {locate, Reason}}
     end.
 
-parse_one_input(#path{orig=In}, Content, #import_env{opts=Opts}) ->
-    FName = file_name_from_input(In),
+parse_one_input(Path, Content, #import_env{opts=Opts}) ->
+    FName = path_to_filename(Path, orig),
     case scan_and_parse_string(Content, FName, Opts) of
         {ok, Defs} ->
             Imports = gpb_defs:fetch_imports(Defs),
@@ -3877,9 +3877,6 @@ add_curr_dir_as_include_if_needed(Opts) ->
         false -> Opts ++ [{i,"."}]
     end.
 
-file_name_from_input({Mod,_S}) -> lists:concat([Mod, ".proto"]);
-file_name_from_input(FName)    -> FName.
-
 scan_and_parse_string(S, FName, Opts) ->
     case gpb_scan:binary(unicode:characters_to_binary(S)) of
         {ok, Tokens, _} ->

test/gpb_compile_tests.erl

@@ -199,6 +199,26 @@ flush_msgs() ->
             []
     end.
 
+several_imports_with_fetcher_test() ->
+    A = <<"import 'b.proto';
+           import 'c.proto';
+           message A { optional uint32 a = 1; }">>,
+    B = <<"message B { optional uint32 b = 2; }">>,
+    C = <<"message C { optional uint32 c = 3; }">>,
+    Fetcher = fun("b.proto") -> {ok, binary_to_list(B)};
+                 ("c.proto") -> {ok, binary_to_list(C)}
+              end,
+    Vsn = gpb_defs:latest_defs_version(),
+    {ok, [{proto_defs_version, Vsn} | Defs], _Warns=[]} =
+        gpb_compile:string(a, binary_to_list(A),
+                           [to_proto_defs,
+                            return_errors, return_warnings,
+                            {import_fetcher, Fetcher},
+                            {proto_defs_version, Vsn}]),
+    ["a.proto", "b.proto", "c.proto"] =
+        [Full || {file, {_BaseSansExt, Full}} <- Defs],
+    ok.
+
 parses_file_to_binary_test() ->
     Contents = <<"message Msg { required uint32 field1 = 1; }\n">>,
     {ok, 'X', Code, []} =