Can't use `@Mapping#source` for map keys containg the dot character

Expected behavior

Using the new map->bean mapping feature it is not possible to use @Mapping#source for keys containing a . character. Instead, the Map type itself seems to be considered as the source type and its methods are proposed as alternatives.

It's still possible to achieve that with expression of course (as in the commented out line in the example), but it's far less elegant as just using the source.

Actual behavior

error: No property named "a.b" exists in source parameter(s). Did you mean "a.bytes"?
        @Mapping(target = "value", source = "a.b")

Steps to reproduce the problem

package com.example;

import org.junit.jupiter.api.Test;
import org.mapstruct.Mapper;
import org.mapstruct.Mapping;
import org.mapstruct.factory.Mappers;

import java.util.Map;

import static org.assertj.core.api.Assertions.assertThat;

class MapstructExampleTest {

    public static record ExampleBean(String value) { }

    @Mapper
    public interface ExampleMapper {

        @Mapping(target = "value", source = "a.b")
        //@Mapping(target = "value", expression = "java(source.get(\"a.b\"))")
        ExampleBean map(Map<String, String> source);
    }

    @Test
    void shouldMapToBean() {
        Map<String, String> source = Map.of("a.b", "test");
        var output = Mappers.getMapper(ExampleMapper.class).map(source);
        assertThat(output.value()).isEqualTo("test");
    }

}

MapStruct Version

1.5.3

Start with the ExampleMapper and shouldMapToBean reproducer, focusing on how @Mapping#source resolves the dotted key "a.b" for a Map<String, String> source. Run the reproducer with MapStruct 1.5.3 and trace the source-property resolution path. Done means the mapping compiles and the test produces an ExampleBean whose value is "test".