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

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.