Improve linting of LHS expressions

[MikhailArkhipov]

Fixes #2068

Original condition was too harsh. Intent was to avoid reporting of variables that are actually being defined, but that can be done by skipping over name expressions in LHS.

[jakebailey]

Seems to fail the NoRightSideCheck test.

My test was something like:

y = {}
invalid.access, another.access, (y.okay, fake.variable.access) = [1, 2, (3, 4)]

But the current code appears to correctly only squiggle invalid, another, and fake.