The `__future__` we relied on is now, where the 3 specific things are
all included [since Python 3.0](https://docs.python.org/3/library/__future__.html):
* absolute_import
* print_function
* unicode_literals
* division
These import statements are no-ops and are no longer necessary.
Replaced dirname() as the man page says (alarmingly) that it might write to
the input string. Also use asprintf() to build the new value for
DYLD_LIBRARY_PATH.
Changed the RemoteMirror test to work around DYLD_LIBRARY_PATH when it's
given an absolute path; when it's called from CI, we really want it to
load the libswiftRemoteMirror.dylib that we just built, *not* the system
one, which is what happens for some of the builders.
rdar://82124292
