Fix 'str' object has no attribute 'dirname'

[freyes]

This stacktrace was found when trying to build the manila charm.

Traceback (most recent call last):
  File "/snap/charm/796/usr/local/bin/charm-build", line 33, in <module>
    sys.exit(load_entry_point('charm-tools==3.0.8.post7+g2463e7a', 'console_scripts', 'charm-build')())
  File "/snap/charm/796/usr/local/lib/python3.10/dist-packages/charmtools/build/builder.py", line 1251, in main
    build()
  File "/snap/charm/796/usr/local/lib/python3.10/dist-packages/charmtools/build/builder.py", line 854, in __call__
    self.generate()
  File "/snap/charm/796/usr/local/lib/python3.10/dist-packages/charmtools/build/builder.py", line 792, in generate
    self.formulate_plan(layers)
  File "/snap/charm/796/usr/local/lib/python3.10/dist-packages/charmtools/build/builder.py", line 598, in formulate_plan
    self.plan = self.plan_layers(layers, output_files)
  File "/snap/charm/796/usr/local/lib/python3.10/dist-packages/charmtools/build/builder.py", line 430, in plan_layers
    wh_tactic.combine(existing_tactic)
  File "/snap/charm/796/usr/local/lib/python3.10/dist-packages/charmtools/build/tactics.py", line 1117, in combine
    self.read()
  File "/snap/charm/796/usr/local/lib/python3.10/dist-packages/charmtools/build/tactics.py", line 1194, in read
    src = path(self.entity.dirname()) / self.CONS_FILENAME
AttributeError: 'str' object has no attribute 'dirname'

Description of change

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• Are all your commits [logically] grouped and squashed appropriately?
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• Does your code pass make test?