Make: Install RTS headers in `$libdir/rts/include` not `$libdir/include`

Before we were violating the convention of every other package. This
fixes that. It matches the changes made in
d5de970d to the location of the files in
the repo.

mk/install.mk.in

@@ -103,7 +103,7 @@ endif
 
 ghclibexecdir = $(ghclibdir)
 topdir        = $(ghclibdir)
-ghcheaderdir  = $(ghclibdir)/include
+ghcheaderdir  = $(ghclibdir)/rts/include
 
 #-----------------------------------------------------------------------------
 # install configuration

rts/package.conf.in

@@ -18,7 +18,7 @@ hidden-modules:
 import-dirs:
 
 #if defined(INSTALLING)
-library-dirs:           LIB_DIR"/rts" FFI_LIB_DIR LIBDW_LIB_DIR
+library-dirs:           LIB_DIR FFI_LIB_DIR LIBDW_LIB_DIR
 #else /* !INSTALLING */
 library-dirs:           TOP"/rts/dist-install/build" FFI_LIB_DIR LIBDW_LIB_DIR
 #endif

rules/manual-package-config.mk

@@ -37,8 +37,8 @@ $1/$2/package.conf.inplace : $1/package.conf.in $$$$(ghc-pkg_INPLACE) | $$$$(dir
 $1/$2/package.conf.install : $1/package.conf.in | $$$$(dir $$$$@)/.
 	$$(HS_CPP) -P \
 		-DINSTALLING \
-		-DLIB_DIR='"$$(if $$(filter YES,$$(RelocatableBuild)),$$$$topdir,$$(ghclibdir))"' \
-		-DINCLUDE_DIR='"$$(if $$(filter YES,$$(RelocatableBuild)),$$$$topdir,$$(ghclibdir))/include"' \
+		-DLIB_DIR='"$$(if $$(filter YES,$$(RelocatableBuild)),$$$$topdir,$$(ghclibdir))/$1"' \
+		-DINCLUDE_DIR='"$$(if $$(filter YES,$$(RelocatableBuild)),$$$$topdir,$$(ghclibdir))/$1/include"' \
 		$$($1_PACKAGE_CPP_OPTS) \
 		$$(addprefix -I,$$(GHC_INCLUDE_DIRS)) \
 		-I$$(BUILD_$3_INCLUDE_DIR) \