allmydataectomy: rename "allmydata" package to "tahoe_lafs" or "tahoelafs"

[warner]

Now that allmydata.com has been dead for like 5 years, it's probably time to remove "allmydata" from the package name. This means:

• replace all imports with relative imports (reducing the code impact of a rename), which is possible because we now require a python version that provides relative imports
• decide on "tahoe_lafs" or "tahoelafs" or "tahoe" or "TahoeLAFS"
• rename the src/allmydata/ directory to match
• update the setup.py metadata to match
• change the name we use for git tags, currently like allmydata-tahoe-1.10, maybe to just "1.10" or "v1.10"
• update the PyPI package, debian/ubuntu packages, etc, if they're using the old allmydata-tahoe name