Correct cd history when going to package not on current PWD

[SuperJappie08]

Corrects the cd jump back behavior to work as expected #27.

[SuperJappie08]

@cottsay, Friendly ping to prevent this PR from being open for many years.

[nuclearsandwich]

My understanding is that this change is here so that if no package is found and thus no change in directory is finalized this restores the shell context back as if when running cd normally you tried to cd into a non-existent directory. It might be worth going through and annotating for future contributors when OLDPWD and the internal _colcon_cd_old_pwd variables are set, updated and unset to make the maintenance easier on future contributors.

[SuperJappie08]

I added some comments. Are they clear enough?

[mjcarroll]

@nuclearsandwich are the comments enough to address your feedback here?

[SuperJappie08]

@cottsay, The current test failure is in an unrelated part of the code.
Should I also try to fix it in this PR?

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[cottsay]

The current test failure is in an unrelated part of the code.
Should I also try to fix it in this PR?

I fixed it in master and merged the fix back into this PR.