requirements-swh.txt: Sort swh.core extra requires

It enables to remove "duplicated" line in pip-swh-packages output.

$ ./bin/pip-swh-packages
-e swh-core
-e swh-model
-e swh-core[db,http]
-e swh-objstorage
-e swh-storage
-e swh-scheduler
-e swh-journal
-e swh-loader-core
-e swh-indexer
-e swh-search
-e swh-vault
-e swh-deposit
-e swh-core[http,db]
-e swh-graph
-e swh-lister
-e swh-loader-git
-e swh-loader-mercurial
-e swh-loader-svn
-e swh-objstorage-replayer
-e swh-scanner
-e swh-web
-e swh-web-client
-e swh-icinga-plugins
-e swh-docs
-e swh-dataset

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Migrated from D4098 (view on Phabricator)