Make MigrateSourceInterface::fields() return the correct values

Problem/Motivation

There are some inconsistencies in fields() method of source plugins. More details:

1) Plugin: d6\ViewMode
Current value:
'display_settings' => $this->t('Serialize data with display settings.'),

And display_settings is not made available at all. The fields made available by ViewMode is an associative array keyed by view mode with two properties, 'entity_type' and 'view_mode'. As in this example:

[
  [entity_type] => node
  [view_mode] => 4
]

2) Plugin: d7\File

1. 'uri' field is available and not described;
2. 'filepath' => $this->t('File path'), is not clear enough what the path is really are
3. If the File Entity → module is installed, then there is another field "type"

3) Plugin: d6\Upload
fid, description, and list keys are not available in the top-level array. They exists as keys of upload sub-arrays.

Proposed resolution

Fix it.

Remaining tasks

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