SI-8907 Don't force symbol info in isModuleNotMethod

Test case by Jason.

RefChecks adds the lateMETHOD flag lazily in its info transformer.
This means that forcing the `sym.info` may change the value of
`sym.isMethod`.

0ccdb151f introduced a check to force the info in isModuleNotMethod,
but it turns out this leads to errors on stub symbols (SI-8907).

The responsibility to force info is transferred to callers, which
is the case for other operations on symbols, too.
I forgot to generalise the prefix `a1` out of existence when I refactored this.