Extract kotlin-reflect-api module out of kotlin-reflect

This is needed only for faster compilation of the Kotlin project itself
and has no effect on the public artifact
org.jetbrains.kotlin:kotlin-reflect.

The problem this is solving is the rebuild of the project once anything
has been changed in modules in 'core' (even inside function bodies, i.e.
a non-API change). Previously, changes in 'core' led to the compilation
of kotlin-reflect, which led to the rebuild of all modules depending on
kotlin-reflect directly or indirectly (which is almost all modules in
the project) because kotlin-reflect's artifacts are custom-built and the
changes can not be picked up incrementally. But 99.9% of the time the
initial changes in 'core' could not have any effect on the usages of
kotlin-reflect, because classes from those modules are moved to an
internal package in kotlin-reflect and thus are an internal
implementation detail.

Now, changes in 'core' still lead to the compilation of kotlin-reflect
and to the process of building the custom jar. But if a module depends
on kotlin-reflect-api, not kotlin-reflect, then the incremental
difference checker will detect that the module does not have to be
recompiled if there hasn't been any changes to the API of
kotlin-reflect-api. Which means that the module will not be rebuilt on
every change in 'core'.

This commit only introduces the new module. The dependencies
(kotlin-reflect -> kotlin-reflect-api) are replaced in the next commit.

settings.gradle

@@ -117,6 +117,7 @@ include ":kotlin-build-common",
         ":kotlin-compiler-embeddable",
         ":kotlin-compiler-client-embeddable",
         ":kotlin-reflect",
+        ":kotlin-reflect-api",
         ":kotlin-ant",
         ":compiler:tests-java8",
         ":compiler:tests-ir-jvm",
@@ -171,6 +172,7 @@ project(':kotlin-stdlib-jdk7').projectDir = "$rootDir/libraries/stdlib/jdk7" as
 project(':kotlin-stdlib-jdk8').projectDir = "$rootDir/libraries/stdlib/jdk8" as File
 project(':kotlin-stdlib:samples').projectDir = "$rootDir/libraries/stdlib/samples" as File
 project(':kotlin-reflect').projectDir = "$rootDir/libraries/reflect" as File
+project(':kotlin-reflect-api').projectDir = "$rootDir/libraries/reflect/api" as File
 project(':kotlin-compiler').projectDir = "$rootDir/prepare/compiler" as File
 project(':kotlin-compiler-embeddable').projectDir = "$rootDir/prepare/compiler-embeddable" as File
 project(':kotlin-compiler-client-embeddable').projectDir = "$rootDir/prepare/compiler-client-embeddable" as File