Prior to any Spack build, Spack modifies PATH etc. to help the build
find the dependencies it needs. It also allows any package to define
custom environment modifications (and furthermore a package can
specify environment modifications to apply when it is used as a
dependency). If an external package defines custom environment
modifications that alter PATH, and the external package is in a merged
or system prefix, then that prefix could "override" the Spack-built
packages.
This commit reorders environment modifications so that PrependPath
actions which expose Spack-built packages override PrependPath actions
for custom environment modifications of external packages.
In more detail, the original order of environment modifications is:
* Modules
* Compiler flag variables
* PATH, CMAKE_PREFIX_PATH, and PKG_CONFIG_PATH for dependencies
* Custom package.py modifications in the following order:
* dependencies
* root
This commit changes the order:
* Modules
* Compiler flag variables
* For each external dependency
* PATH, CMAKE_PREFIX_PATH, and PKG_CONFIG_PATH modifications
* Custom modifications
* For each Spack-built dependency
* PATH, CMAKE_PREFIX_PATH, and PKG_CONFIG_PATH modifications
* Custom modifications
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