Allow disabling serving of metadata files

The server will now accept a --no-metadata flag, which will disable serving of metadata files
when provided. This can be useful if installing packages via pip but not providing normalized
names (e.g. pip install FastAPI instead of pip install fastapi). This is to workaround an issue
with pip: when it is provided a non-normalized name, it sends it to the server normalized, but
expects the metadata file returned to have the non-normalized name. However, the server
cannot know what the non-normalized name is. The only way to work around this is to disable
serving of metadata files altogether.