Spack environment updates take precedence (#9107)

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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