Clamp down on dependencies:
Previously:
- "build-depends" entries on internal libraries with an explicit version
range are an error.
Now for all three of "build-tools", "tool-depends", and "build-depends":
- Explicit versions ranges on internal components that are satisfied by
the current package cause a warning.
- Explicit version ranges on internal components are are not satisfied by
the current package are an error.
And additionally for "tool-depends":
- dependencies on a non-existent executable in the current package are an
error.