Move file export

[renefloor]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix similar to #321

⤵️ What is the current behavior?

File is always imported with the library conflicting with dart:io

🆕 What is the new behavior (if this is a feature change)?

Give the user of the library a different way to import the file class

💥 Does this PR introduce a breaking change?

It reverts a breaking change

🐛 Recommendations for testing

📝 Links to relevant issues/docs

Fixes #320

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[renefloor]

@sidrao2006, @akadatsky and @aideric.
What do you think of this solution? This way the default import

import 'package:flutter_cache_manager/flutter_cache_manager.dart';

won't conflict with dart:io, but you'll still have the option to add an import with

import 'package:flutter_cache_manager/file.dart';

[sidrao2006]

Yep, this is better than having to add file as a direct dependency

[codecov]

Codecov Report

Merging #324 (11c6dc5) into develop (b1e9807) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #324   +/-   ##
========================================
  Coverage    74.85%   74.85%           
========================================
  Files           21       21           
  Lines          672      672           
========================================
  Hits           503      503           
  Misses         169      169           

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