Extract API subpackage

[thisac]

Context: Due to the XCC integration, most of the Xanadu API subpackage is now gone.

Description of the Change:

• strawberryfields.api.results.py and strawberryfields.api.devicespec.py are moved to the root strawberryfields folder, and the strawberryfields.api folder is removed.
• The FailedJobError is moved from strawberryfields.api.__init__.py to strawberryfields.engine.py (where it's used).

Benefits: Things look cleaner.

Possible Drawbacks: None

Related GitHub Issues: None

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (integrate-xanadu-cloud-client@da47407). Click here to learn what that means. The diff coverage is n/a.

@@                       Coverage Diff                        @@
##             integrate-xanadu-cloud-client     #652   +/-   ##
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  Coverage                                 ?   98.27%           
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  Files                                    ?       72           
  Lines                                    ?     8811           
  Branches                                 ?        0           
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  Hits                                     ?     8659           
  Misses                                   ?      152           
  Partials                                 ?        0           

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