[BUGFIX][EM] Get extension show spinner icon in upper right table

The load mask logic in extension manager stumbles upon the fluid change
of issue #40826 and now always shows a spinner in the "Get extensions"
tab.
This is solved with a small cleanup of the classes and markup added to
this HTML section which anyway are obsolete since #46029.

Change-Id: I0a2e5b26c890ba7ba93bf4abb1f6485084ebf808
Resolves: #46455
Related: #46029
Related: #40826
Releases: 6.1
Reviewed-on: https://review.typo3.org/19097
Reviewed-by: Jost Baron
Tested-by: Jost Baron
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Reviewed-by: Alexander Schnitzler
Tested-by: Alexander Schnitzler
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
[BUGFIX] ExtDirectApi uses an undefined variable

Method generateAPI() sets $className inside the condition but later uses
it without checking if it is empty or not. The result is a PHP warning
"Invalid argument supplied for foreach()".

Change-Id: I7fd2bf88374661a8b54023aee9fe2af5619471b0
Resolves: #46075
Releases: 6.0, 6.1
Reviewed-on: https://review.typo3.org/18760
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
[BUGFIX] Fix .zip-export on windows and add unit test

On Windows servers (xampp), the .zip-export of extensions did not
work because \ZipArchive::addFile can't add directories on Windows.

Added a unit test for the export function.

Fixes: #45140
Releases: 6.0, 6.1
Change-Id: If248bf031d3d7b621971de609b86eedc0ee05a64
Reviewed-on: https://review.typo3.org/18961
Reviewed-by: Jost Baron
Tested-by: Jost Baron
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn