[FIX] Add-ons: Fix Installation of Official Add-ons Through Drag & Drop

[nikicc]

Issue

Installing official add-ons through drag & drop doesn't work from cleaning the add-ons names onwards (3.4.0 onwards?). The mismatch occurs when we compare cleaned addon name with the zip file name. Since the official addon name was cleaned it doesn't match the uncleaned zip filename.

Description of changes

Clean the drag & drop file name in the same manner as official addons.

Includes

• Code changes
• Tests
• Documentation

[kernc]

```py
future.set_result((AddonManagerDialog._packages or []) + packages)
```
to
```py
future.set_result(packages + (AddonManagerDialog._packages or []))
```
~~~a few lines below, so as to sort the dropped package first, _this_ will be the one found and selected when multiple packages with the same name are on the list (i.e. a package listed from PyPi and the a more recent version of it dropped).~~~

~~~OTOH, this might not be necessary, both matching items are selected and likely both get installed, the more recent item overwriting.~~~

nvm.

[codecov-io]

Codecov Report

Merging #2481 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2481      +/-   ##
==========================================
+ Coverage   74.51%   74.53%   +0.02%     
==========================================
  Files         321      321              
  Lines       56056    56056              
==========================================
+ Hits        41769    41784      +15     
+ Misses      14287    14272      -15