chore: Remove ServiceLoader use from the codebase (#705)

Score Directors are now accessed directly, 
as we no longer have the need to be able to exclude some of them from the classpath.
Joiners for CS are also accessed directly, for the same reason. 
Enterprise Edition no longer uses ServiceLoader and instead uses reflection.

Because of this, core is no longer 3 Maven modules.
core, core-impl and constraint-streams modules were all folded into core,
which is now a top-level module.

All the changes are either file renames to different places and packages,
or changes strictly necessary to get rid of ServiceLoader.
There are no unrelated changes in this PR.

benchmark/pom.xml

@@ -42,7 +42,7 @@
     </dependency>
     <dependency>
       <groupId>ai.timefold.solver</groupId>
-      <artifactId>timefold-solver-core-impl</artifactId>
+      <artifactId>timefold-solver-core</artifactId>
       <type>test-jar</type>
       <scope>test</scope>
     </dependency>
@@ -238,11 +238,9 @@
         <artifactId>maven-dependency-plugin</artifactId>
         <configuration>
           <ignoredUsedUndeclaredDependencies>
-            <dependency>ai.timefold.solver:timefold-solver-core-impl:jar</dependency>
             <dependency>org.webjars:*:jar</dependency>
           </ignoredUsedUndeclaredDependencies>
           <ignoredUnusedDeclaredDependencies>
-            <dependency>ai.timefold.solver:timefold-solver-core:jar</dependency>
             <dependency>org.webjars:*:jar</dependency>
           </ignoredUnusedDeclaredDependencies>
         </configuration>

build/bom/pom.xml

@@ -38,35 +38,13 @@
       </dependency>
       <dependency>
         <groupId>ai.timefold.solver</groupId>
-        <artifactId>timefold-solver-core-impl</artifactId>
-        <version>${version.ai.timefold.solver}</version>
-      </dependency>
-      <dependency>
-        <groupId>ai.timefold.solver</groupId>
-        <artifactId>timefold-solver-core-impl</artifactId>
-        <version>${version.ai.timefold.solver}</version>
-        <classifier>sources</classifier>
-      </dependency>
-      <dependency>
-        <groupId>ai.timefold.solver</groupId>
-        <artifactId>timefold-solver-core-impl</artifactId>
-        <type>test-jar</type>
-        <version>${version.ai.timefold.solver}</version>
-      </dependency>
-      <dependency>
-        <groupId>ai.timefold.solver</groupId>
-        <artifactId>timefold-solver-constraint-streams</artifactId>
-        <version>${version.ai.timefold.solver}</version>
-      </dependency>
-      <dependency>
-        <groupId>ai.timefold.solver</groupId>
-        <artifactId>timefold-solver-constraint-streams</artifactId>
+        <artifactId>timefold-solver-core</artifactId>
         <version>${version.ai.timefold.solver}</version>
         <classifier>sources</classifier>
       </dependency>
       <dependency>
         <groupId>ai.timefold.solver</groupId>
-        <artifactId>timefold-solver-constraint-streams</artifactId>
+        <artifactId>timefold-solver-core</artifactId>
         <type>test-jar</type>
         <version>${version.ai.timefold.solver}</version>
       </dependency>