We were using the omission of toolchain from the MVS graph
as a signal that toolchain was not mentioned on the go get line,
but not including it in the graph causes various problems,
and it may be reintroduced to the graph during operations like
pruning conversion, after which its presence is not a good signal
about whether it was mentioned on the go get command line.
Fix all this irregularity by explicitly telling WriteGoMod whether
the command line mentioned toolchain instead.