Handle duplicated files for validation

[roll]

Overview

Configurations like this could lead to duplication of files for validation:

files: *
datapackages:
  - datapackage.json

So we could:

• describe it in Docs/FAQ
• handle on GTIO level (datapackage files have priority)
• handle on goodtables level (datapackage files have priority)
• handle on GTIO level based on like settings.unique: true
• handle on goodtables-py level (may be also based on inspector setting)
• use warning (in report) system of goodtables-py - https://github.com/frictionlessdata/goodtables-py/issues/157
• ???

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For Beta files/datapackages are mutually exclusive (#171).

[amercader]

To be perfectly honest I would make our life easier for the Beta release and make them exclusive (ie if you have files, datapackages is ignored or viceversa).