Split core skylarkbuildapi types into a new package.

This allows subpackages (such as platform) to only depend on the parts they need.

This is needed for https://github.com/bazelbuild/bazel/issues/10368.

PiperOrigin-RevId: 284184159
diff --git a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/SkylarkRuleContextApi.java b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/SkylarkRuleContextApi.java
index 0970080..4927994 100644
--- a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/SkylarkRuleContextApi.java
+++ b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/SkylarkRuleContextApi.java
@@ -17,6 +17,9 @@
 import com.google.common.collect.ImmutableList;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.events.Location;
+import com.google.devtools.build.lib.skylarkbuildapi.core.ProviderApi;
+import com.google.devtools.build.lib.skylarkbuildapi.core.StructApi;
+import com.google.devtools.build.lib.skylarkbuildapi.core.TransitiveInfoCollectionApi;
 import com.google.devtools.build.lib.skylarkinterface.Param;
 import com.google.devtools.build.lib.skylarkinterface.ParamType;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkCallable;