flatmap: mark computed list as a computed value in Expand

[mitchellh]

Fixes #12183

The fix is in flatmap for this but the entire issue is a bit more
complex. Given a schema with a computed set, if you reference it like
this:

lookup(attr[0], "field")

And "attr" contains a computed set within it, it would panic even though
"field" is available. There were a couple avenues I could've taken to
fix this:

1.) Any complex value containing any unknown value at any point is
entirely unknown.

2.) Only the specific part of the complex value is unknown.

I took route 2 so that the above works without any computed (since
"name" is not computed but something else is). This may actually have an
effect on other parts of Terraform configs, however those similar
configs would've simply crashed previously so it shouldn't break any
pre-existing configs.

Given the fix, I don't recommend this be backported.

[jbardin]

impressive fix/test ratio ;)

[mitchellh]

Hah, there were a number of places where this could go wrong :P Wanted to test each

[ghost]

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