Externals Updates, master branch (2020.10.27.)

Updated all projects to atlasexternals-2.0.83. The changes wrt. atlasexternals-2.0.82 are the following (atlasexternals@2.0.82...2.0.83):

• Added Findimportlib_metadata.cmake and FindSQLite3.cmake to AtlasLCG;
• Updated flake8 to version 3.8.4 (thanks to @fwinkl);
• Updated the CheckerGccPlugins code to take certain function attributes correctly into account (thanks to @ssnyder);
• Added stomp.py as a new external to Externals/PyModules (thanks to @ponyisi);
• Updated the default path that LCG nightlies would be searched under (thanks to @simmons);
• Added various updates for ARM 64 build compatibility (as part of ATLINFR-3794).

Note that the updates are all incremental build friendly. But just to make sure that stomp.py is cleaned off of the CI nodes until this MR is accepted, I still added the full-build label.