This code fixes a bug when a user specifies NeedTypes, which
implicitly requires TypesSizes, but TypesSizes isn't fetched,
which causes typechecking to explode.
Also fix a similar issue where NeedImports isn't implicitly
fetched for NeedDeps.
I added a TODO for a better fix, which is to have an "implicitMode"
in the loader type containing all the data that's needed as a prerequisite
for other fields. Then we can use implicitMode when fetching data,
and cfg.Mode to clear out the fields the user didn't request.
Fixesgolang/go#31163
Change-Id: If3506765470af43dfb24d06fcbd31b66a623f2e0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/170342
Reviewed-by: Ian Cottrell <iancottrell@google.com>
1aadbdfdbb