Replace WalkBuilder.add_ignore with WalkBuilder.overrides

[prantlf]

Fix the support of files.exclude from .denolint.json.

It did not work at all. Neither with a directory, nor with a patern. Printing the errors from add_ignore revealed it:

lib/: line 1: Is a directory (os error 21)
lib/*.js: No such file or directory (os error 2)

Overrides are supposed to handle both directories and patterns.

Attempts to fix half of #635. The second half of the fix is #645.