Adjustments:
 * renamed src to hgext/narrow
 * marked extension experimental
 * added correct copyright header where it was missing
 * updated hgrc extension enable line in library.sh
 * renamed library.sh to narrow-library.sh
 * dropped all files from repo root as they're not interesting
 * dropped test-pyflakes.t, test-check-code.t and test-check-py3-compat.t
 * renamed remaining tests to all be test-narrow-* when they didn't already
 * fixed test-narrow-expanddirstate.t to refer to narrow and not narrowhg
 * fixed tests that wanted `update -C .` instead of `merge --abort`
 * corrected a two-space indent in narrowspec.py
 * added a missing _() in narrowcommands.py
 * fixed imports to pass the import checker
 * narrow only adds its --include and --exclude to clone if sparse isn't
   enabled to avoid breaking test-duplicateoptions.py. This is a kludge,
   and we'll need to come up with a better solution in the future.

These were more or less the minimum to import something that would
pass tests and not create a bunch of files we'll never use.

Changes I intend to make as followups:
 * rework the test-narrow-*-tree.t tests to use the new testcases
   functionality in run-tests.py
 * remove lots of monkeypatches of core things

Differential Revision: https://phab.mercurial-scm.org/D1974