charm compose collides with libs when they are shared among layers

[Charles Butler]

When building a charm from resulting compose layers that appear to have shared dependencies, composer fails w/ "library already exists"

http://paste.ubuntu.com/12254981/

Relevent snippet from the above debug log:

utils: process: pip install -U --exists-action i -t /tmp/tmpmb9Jra charmhelpers>=0.4.0,<1.0.0 (0)
Traceback (most recent call last):
  File "/home/charles/.virtualenv/bin/charm-compose", line 9, in <module>
    load_entry_point('charm-tools==1.6.1', 'console_scripts', 'charm-compose')()
  File "/home/charles/.virtualenv/local/lib/python2.7/site-packages/charmtools/compose/__init__.py", line 462, in main
    composer()
  File "/home/charles/.virtualenv/local/lib/python2.7/site-packages/charmtools/compose/__init__.py", line 391, in __call__
    self.generate()
  File "/home/charles/.virtualenv/local/lib/python2.7/site-packages/charmtools/compose/__init__.py", line 357, in generate
    self.exec_plan(self.plan, self.layers)
  File "/home/charles/.virtualenv/local/lib/python2.7/site-packages/charmtools/compose/__init__.py", line 335, in exec_plan
    tactic()
  File "/home/charles/.virtualenv/local/lib/python2.7/site-packages/charmtools/compose/tactics.py", line 417, in __call__
    d.move(target)
  File "/usr/lib/python2.7/shutil.py", line 292, in move
    raise Error, "Destination path '%s' already exists" % real_dst
shutil.Error: Destination path '/home/charles/tmp/charms/trusty/docker-nginx/lib/markupsafe' already exists

This appears to be an issue with the default tactic colliding on a dependent file that is surfaced from the "base" layer and causes the composition to fail.