cmd/go: allow 'go generate' even if imports do not resolve
Maybe the go generate is generating the imports,
or maybe there's some other good reason the code
is incomplete.
The help text already says:
Note that go generate does not parse the file, so lines that look
like directives in comments or multiline strings will be treated
as directives.
We'll still reject Go source files that don't begin with a package statement
or have a syntax error in the import block, but those are I think more
defensible rejections.