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Fix substitution problems introduced in 2.8 #599
The way #515 was implemented did not play nicely with env var substitution. This might render the former additions completely or partially obsolete but I did not check this yet. First let's fix that bug.
Instead of crashing early on testenv creation if substitution key is missing, the missing keys are added to a list attached to TestenvConfig. This way it is not necessary anymore to provide everything needed to resolve all testenvs but instead only what is needed to resolve all testenvs that are part of this restrun.
The first implementation used a module level mapping from envname to missing substitutions, which was ugly. Instead the information is bubbled up as an exception where it can be handled the way it is needed in the contexts where the information can be either translated into a crashing error or attached to the object which should carry the information (in this case TestenvConfig).
Atm a missing substitution in a testenv is not being left empty but filled with a special value 'TOX_MISSING_SUBSTITUTION', this is not exactly necessary, but I don't feel comfortable enough yet to leave it empty, this can help with debugging problems that might arise from this change.