Fix module names in requirements.

Remove the depth-one tree in py-deps-swh.svg doc as the root.

requirements-swh.txt

 # Add here internal Software Heritage dependencies, one per line.
-swh-core
-swh-model
-swh-objstorage[testing]
-swh-scheduler
-swh-storage[schemata,listener]
-swh-loader-core
-swh-lister
-swh-journal
-swh-vault
-swh-loader-dir
-swh-loader-tar
-swh-loader-pypi
-swh-loader-debian
-swh-loader-mercurial
-swh-loader-svn
-swh-loader-git
-swh-archiver
-swh-web
-swh-deposit
-swh-indexer
-swh-mirror-forge
+swh.core
+swh.model
+swh.objstorage[testing]
+swh.scheduler
+swh.storage[schemata,listener]
+swh.loader.core
+swh.lister
+swh.journal
+swh.vault
+swh.loader.dir
+swh.loader.tar
+swh.loader.pypi
+swh.loader.debian
+swh.loader.mercurial
+swh.loader.svn
+swh.loader.git
+swh.archiver
+swh.web
+swh.deposit
+swh.indexer
+swh.mirror.forge