Make buildgrid namespace. Move sources to main package

In order to use buildgrid-common in buildgrid, we will need to convert the toplevel buildgrid package into a namespace. I have moved all of the modules to a "main" package and then did a find/replace to update contents.

Steps taken:

mkdir main
mv buildgrid/* main
mv main buildgrid

In VS Code:

Replace: ([^./]|(?:[.]/))buildgrid([./])((_app)|(_enums)|(_exceptions)|(_protos)|(_types)|(_version)|(bot)|(browser)|(cleanup)|(client)|(server)|(settings)|(utils))

With: $1buildgrid$2main$2$3

Exclude: *.proto,*pb2.py,*.pyi,*aio.py

tox -e protos
tox -e format