Previously, the generated resources were added as an output to the
main source set. This worked on the command line, but resulted in
the META-INF folder that contains the loader jar not being on the
classpath of downstream projects in Eclipse.

This commit changes loader-tools to add the generated resources
as a srcDir to the main source set. This results in it appearing on
the classpath of other projects in Eclipse that depend on loader-tools
such as the Gradle plugin, thereby allowing its tests to be run in
the IDE as well as on the command line.

Fixes gh-19841