pager: move pager.[ch] src/shared/ → src/basic/

pager.[ch] doesn't use any APIs from src/libsystemd/ or src/shared/
hence there's no reason for it to be in src/shared/, let's move it to
src/basic/ instead.

This enables us to use pager.[ch] APIs from other code in src/basic/,
for example pager_have() and suchlike.

src/basic/meson.build

@@ -127,6 +127,8 @@ basic_sources = files('''
         nss-util.h
         ordered-set.c
         ordered-set.h
+        pager.c
+        pager.h
         parse-util.c
         parse-util.h
         path-util.c

src/shared/meson.build

@@ -69,8 +69,6 @@ shared_sources = '''
         nsflags.h
         output-mode.c
         output-mode.h
-        pager.c
-        pager.h
         path-lookup.c
         path-lookup.h
         ptyfwd.c