core: Encode Targets in saved Planfile

[phinze]

When a user specifies -targets on a terraform plan and stores
the resulting diff in a plan file using -out - it usually works just
fine since the diff is scoped based on the targets.

When there are tainted resources in the state, however, graph nodes to
destroy them were popping back into the plan when it was being loaded
from a file. This was because Targets weren't being stored in the
Planfile, so Terraform didn't know to filter them out. (In the
non-Planfile scenario, we still had the Targets loaded directly from the
flags.)

By encoding Targets in with the Planfile we can ensure that the same
filters are always applied.

Backwards compatibility should be fine here, since we're just adding a
field. The gob encoder/decoder will just do the right thing (ignore/skip
the field) with planfiles stored w/ versions that don't know about
Targets.

Fixes #5183

[phinze]

ping @jen20 and/or @mitchellh for review here

[jen20]

LGTM - just ran the tests locally and all appears to be well.

[ghost]

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