Thrift build command import modification isn't idempotent

[spladug]

https://github.com/reddit/baseplate.py/blob/master/baseplate/frameworks/thrift/command.py#L55

before:

from foo.foo_thrift import FooService

run 1:

from foo.foo_thrift.foo_thrift import FooService

run 2:

from foo.foo_thrift.foo_thrift.foo_thrift import FooService

etc.