Use export_files to export the KSP JAR files (#1313)

This fixes the visibility error for "symbol-processing-cmdline.jar" in
#1193.

The legacy behavior of Bazel is that input source files are implicitly
exported with the `default_visibility` set in the `BUILD` file.  The
`--incompatible_no_implicit_file_export` option disables this legacy
behavior, making all input source files private by default, and the
Bazel documentation recommends that files should always be exported
explicitly if they are needed outside the current package:

> Avoid relying on the legacy behavior. Always write an `exports_files`
> declaration whenever a source file target needs non-private visibility.

(see https://bazel.build/concepts/visibility#source-file-target-visibility)

Author: kpitt

src/main/starlark/core/repositories/BUILD.com_github_google_ksp.bazel

@@ -13,8 +13,16 @@
 # limitations under the License.
 package(default_visibility = ["//visibility:public"])
 
-# KSP filegroup containing everything that might be needed
+# Collect list of all KSP files that might be needed
+_KSP_FILES = glob(["**"])
+
+# KSP filegroup containing all KSP files to export
 filegroup(
     name = "ksp",
-    srcs = glob(["**"]),
+    srcs = _KSP_FILES,
 )
+
+# If the `--incompatible_no_implicit_file_export` option is enabled, then the
+# individual "srcs" from the "ksp" filegroup will not be visible, so also
+# export them with `export_files` to ensure that they are made public.
+exports_files(_KSP_FILES)