Check missing uncoditionally loaded missing modules at runtime

A potentially inherited (via dynamically calculated path) module may in turn
unconditionally load a module that does no exist in a source tree -- it is
not an error if this potentially inherited module is actually never inherited
because its dynamically calculated path will never reference it. Instead of
emitting an uncoditional `load` for a non-existent file (which is going to fail
in the Starlark parser), emit conditional load and a runtime check.

Fixes: 213922819
Test: internal
Change-Id: I92177878e199a1f00e5f0c4045c0c0daeddd6bdb