Target not found from subrepo

[peterebden]

I added the following to a repo (specifically thought-machine/please-servers):

github_repo(
    name = "please",
    repo = "thought-machine/please",
    revision = "v17.8.0",
)

but it won't build with

$ plz build ///third_party/please//tools/sandbox:sandbox
Building [0/1, 0.0s]:
  CPU use:   0.0%  I/O:   0.0%  Mem use:   0.0%
13:43:19.282 CRITICAL: Target ///cc//build_defs:cc not found in build graph

However that target does exist:

$ plz build ///cc//build_defs:cc
Build finished; total time 750ms, incrementality 100.0%. Outputs:
///cc//build_defs:cc:
  plz-out/gen/cc/build_defs/cc.build_defs