Spack originally handled environment modifications in the following
order:
1. clear environment variables
(unless Spack was invoked with --dirty)
2. apply spack-specific environment variable updates,
including variables set by Spack core like CC/PKG_CONFIG_PATH
and those set by installed dependencies (e.g. in
setup_dependent_environment)
3. load all external/compiler modules
1 and 2 were done together. This splits 1 into its own function and
imposes the following order for environment modifications:
1. clear environment variables
2. load all external/compiler modules
3. apply spack-specific environment variable updates
As a result, prepend-path actions taken by Spack (or installed Spack
dependencies) take precedence over prepend-path actions from compiler
and external modules. Additionally, when Spack (or a package
dependency) sets/unsets an environment variable, that will override
the actions of external/compiler modules.
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