Don't add targets that can't be found in GHCi

When using the :add command in haxlsh/ghci, a module/file that can't
be found is still added to the list of targets, resulting in an error
message for the bad module/file for every subsequent usage of the
command. The add command should verify that the module/file can be
found before adding it to the list of targets.

Also add a ":show targets" command to show the currently added list of
commands, and an ":unadd" command to remove a target.

Test Plan:
Add a new GHCi testcase that checks that :add doesn't remember either
files or modules that could not be found, and that both the new :show
and :unadd commands work as expected.

Reviewers: simonmar, bgamari

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14676

Differential Revision: https://phabricator.haskell.org/D4321

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