Move source files to standard locations

[mrjameshamilton]

Some of the sub-projects contained custom sourceSets blocks. This PR removes the custom blocks and moves the files into the standard locations used by the Gradle Java plugin.

I noticed this when adding trying to add some tests to these modules with Diffblue Cover: the tests would normally go in src/test/java/ but src/ was set as the main source set dir in the custom block, so the test source set + main source set locations overlapped.

The contents of the jars are the same before and after these changes:

Archive:  lib-master/annotations-7.6.2.jar		      |	Archive:  lib-branch/annotations-7.6.2.jar
  Length      Date    Time    Name				  Length      Date    Time    Name
---------  ---------- -----   ----				---------  ---------- -----   ----
...
---------                     -------				---------                     -------
     5960                     17 files				     5960                     17 files

Archive:  lib-master/proguard-ant.jar			      |	Archive:  lib-branch/proguard-ant.jar
  Length      Date    Time    Name				  Length      Date    Time    Name
---------  ---------- -----   ----				---------  ---------- -----   ----
...
---------                     -------				---------                     -------
 23687115                     6496 files			 23687115                     6496 files

Archive:  lib-master/proguardgui.jar			      |	Archive:  lib-branch/proguardgui.jar
  Length      Date    Time    Name				  Length      Date    Time    Name
---------  ---------- -----   ----				---------  ---------- -----   ----
...
---------                     -------				---------                     -------
 19460432                     5351 files			 19460432                     5351 files

Archive:  lib-master/retrace.jar			      |	Archive:  lib-branch/retrace.jar
  Length      Date    Time    Name				  Length      Date    Time    Name
---------  ---------- -----   ----				---------  ---------- -----   ----
...
---------                     -------				---------                     -------
 19109962                     5239 files			 19109962                     5239 files

Full diffs:

jar-diff-retrace.txt
jar-diff-ant.txt
jar-diff-gui.txt
jar-diff-annotations.txt

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud