[libc++][random] Removes transitive includes.

It seems these includes are still provided by the sub headers, so it only
removes the duplicates.

There is no change in the list of includes, but the change affects the
modular build. By not having the includes in the top-level header the
module map has changed. This uncovers missing includes in the tests
and missing exports in the module map. This causes the huge amount of
changes in the patch.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D133252

NOKEYCHECK=True
GitOrigin-RevId: 24e1736d84fd0fb45097245706a523c3398beb69
diff --git a/include/module.modulemap.in b/include/module.modulemap.in
index cf1c7dd..969b6b5 100644
--- a/include/module.modulemap.in
+++ b/include/module.modulemap.in
@@ -938,7 +938,10 @@
       module clamp_to_integral               { private header "__random/clamp_to_integral.h" }
       module default_random_engine           { private header "__random/default_random_engine.h" }
       module discard_block_engine            { private header "__random/discard_block_engine.h" }
-      module discrete_distribution           { private header "__random/discrete_distribution.h" }
+      module discrete_distribution {
+        private header "__random/discrete_distribution.h"
+        export vector
+      }
       module exponential_distribution        { private header "__random/exponential_distribution.h" }
       module extreme_value_distribution      { private header "__random/extreme_value_distribution.h" }
       module fisher_f_distribution           { private header "__random/fisher_f_distribution.h" }
@@ -955,8 +958,14 @@
       module mersenne_twister_engine         { private header "__random/mersenne_twister_engine.h" }
       module negative_binomial_distribution  { private header "__random/negative_binomial_distribution.h" }
       module normal_distribution             { private header "__random/normal_distribution.h" }
-      module piecewise_constant_distribution { private header "__random/piecewise_constant_distribution.h" }
-      module piecewise_linear_distribution   { private header "__random/piecewise_linear_distribution.h" }
+      module piecewise_constant_distribution {
+        private header "__random/piecewise_constant_distribution.h"
+        export vector
+      }
+      module piecewise_linear_distribution {
+        private header "__random/piecewise_linear_distribution.h"
+        export vector
+      }
       module poisson_distribution            { private header "__random/poisson_distribution.h" }
       module random_device                   { private header "__random/random_device.h" }
       module ranlux                          { private header "__random/ranlux.h" }