Throw an error for a member reference that refers to different members over time

[nex3]

With @import, it's possible for a file to refer to a mixin or function that gets redefined later on. Right now References will just override the previous definition and treat it as though it always referred to whatever the last value was, but the more correct behavior would be to throw an error, since changing the definition isn't supported in the module system.

For example:

<==> input/entrypoint.scss
@import "definition1";
@import "upstream";
@import "midstream";

<==> input/definition1.scss
@function fn() {@return 1}

<==> input/definition2.scss
@function fn() {@return 2}

<==> input/midstream.scss
@import "definition2";
@import "upstream";

<==> input/upstream.scss
a {b: fn()}

I don't think this is worth fixing before we launch, though.

[nex3]

Given that we're getting ready to start properly deprecating @import and this doesn't seem to be causing much user pain, I'm going to close it out as not planned.

[nex3]

Oops, thought this was an issue on the implementation. For the migrator, we probably should handle this.