Fix composer autoload

[thomas-topway-it]

Fixes https://github.com/SemanticMediaWiki/SemanticMetaTags/issues/76

This file is part of the SemanticMetaTags extension, it is not a valid entry point

• Moves extension autoload from composer.json to extension.json
• Remove redundant semantic-media-wiki require to allow running composer in the extension folder

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.28%. Comparing base (4f1cce3) to head (b9a38a1).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #79 +/- ## ========================================= Coverage 97.28% 97.28% Complexity 67 67 ========================================= Files 6 6 Lines 184 184 ========================================= Hits 179 179 Misses 5 5 ```

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[kghbln]

@thomas-topway-it Is this still a work in progress? If not, your changes should be reviewed and merged for a new release.

[thomas-topway-it]

I'm sorry, I have to fix the tests before

[kghbln]

No worries.

[thomas-topway-it]

so now it should work. @kghbln and @gesinn-it-gea are you able to test/check it together with https://github.com/SemanticMediaWiki/SemanticMetaTags/pull/80 ? once that the 2 PRs will be merged or verified, we could also restore the loading through composer, also if in my opinion is not necessary

[kghbln]

I currently do not have the time to test.