g.extension: refactor/simplify 'GitAdapter' class '_get_addons_list' method

[tmszi]

Describe the bug
The installation of some extension via the g.extension module failed.

To Reproduce
Steps to reproduce the behavior:

1. Try install some extension g.extension db.join
2. See error

Traceback (most recent call last):
  File "/usr/lib64/grass83/scripts/g.extension", line 2876, in <module>
    sys.exit(main())
  File "/usr/lib64/grass83/scripts/g.extension", line 2856, in main
    install_extension(source=source, url=url, xmlurl=xmlurl, branch=branch)
  File "/usr/lib64/grass83/scripts/g.extension", line 1239, in install_extension
    ) = install_extension_std_platforms(
  File "/usr/lib64/grass83/scripts/g.extension", line 2000, in install_extension_std_platforms
    srcdir = download_source_code(
  File "/usr/lib64/grass83/scripts/g.extension", line 1912, in download_source_code
    directory = download_source_code_official_github(
  File "/usr/lib64/grass83/scripts/g.extension", line 1778, in download_source_code_official_github
    ga = GitAdapter(
  File "/usr/lib64/grass83/scripts/g.extension", line 252, in __init__
    self.addons = self._get_addons_list()
  File "/usr/lib64/grass83/scripts/g.extension", line 387, in _get_addons_list
    addons_dict[file_path.split("/")[3]] = "/".join(
IndexError: list index out of range

Expected behavior
The installation of some extension via g.extension module should work.

System description:

• Operating System: UNIX like OS
• GRASS GIS version 8.3.dev

Additional context
Bug was introduced by addons PR OSGeo/grass-addons#887.
To be backported with #2895.

[ninsbl]

Thanks @tmszi for the quick fix and improvement of the related code! Looks good to me.

And sorry for introducing the issue!

Maybe we should run the g.extension tests also in the addons repository, probably in a modified form that tests compilation. That way this issue would have been caught earlier (and maybe even befor hitting users).

[ninsbl]

For the record, the issue was introduced with: OSGeo/grass-addons#887 which added missing Makefiles in sum subdirectories.

[tmszi]

And sorry for introducing the issue!

Bugs happen and appear, this is how we quickly discovered the weak point of this method's code and improve it.

Maybe we should run the g.extension tests also in the addons repository, probably in a modified form that tests compilation. That way this issue would have been caught earlier (and maybe even befor hitting users).

Good idea I agree.