* from_sourcing_file: fixed a bug + added a few ignored variables
closes#7536
Credits for this change goes to mgsternberg (original author of #7536)
The new variables being ignored are specific to Modules v4.
* Use Spack Executable in 'EnvironmentModifications.from_sourcing_file'
Using this class avoids duplicating lower level logic to decode
stdout and handle non-zero return codes
* Extracted a function that returns the environment after sourcing files
The logic in `EnvironmentModifications.from_sourcing_file` has been
simplified by extracting a function that returns a dictionary with the
environment one would have after sourcing the files passed as argument.
* Further refactoring of EnvironmentModifications.from_sourcing_file
Extracted a function that sanitizes a dictionary removing keys that are
blacklisted, but keeping those that are whitelisted. Blacklisting and
whitelisting can be done on literals or regex.
Extracted a new factory that creates an instance of
EnvironmentModifications from a diff of two environments.
* Added unit tests
* PS1 is blacklisted + more readable names for some variables
2fe1ecbaa2