Undefined reference to .o files in subdir when compiling

Question

My project architecture :

project_folder/
 |--bin/
 |--src/
 |    |-----module1/
 |    |      |----> module1.c
 |    |      |----> module1.h
 |    |      |----> module1.o
 |    |      |----> makefile
 |    |-----module2/
 |    |      |----> module2.c
 |    |      |----> module2.h
 |    |      |----> module2.o
 |    |      |----> makefile
 |-- makefile
 |-- main.c

When I make my project It correctly builds the modules of my project and then when I want to compile the whole project I have undefined reference to modules' functions.

I think the error comes from my CC command in my head makefile so here is my makefile :

export CC = gcc

PACKAGES = module1 module2

MAIN = main.c

EXEC = bin

all: $(PACKAGES) $(EXEC)

$(PACKAGES):
    @$(MAKE) $(MAKECMDGOALS) -C $@

$(EXEC): $(MAIN)
    $(CC) $(MAIN) -o $(EXEC)

.PHONY: all clean $(PACKAGES)

clean: $(PACKAGES)

Error is :

/usr/bin/ld: /project_folder/main.c:line undefined reference to `module1_fct1`

Do y have any advice ?