* default scope for config command is made consistent with cmd/__init__ default
* dont specify a scope when looking for compilers with a matching spec (since compiler concretization is scope-independent)
* config edit should default to platform-specific file only for compilers
* when duplicate compiler specs are detected, the exception raised now points the user to the files where the duplicates appear
* updated error message to emphasize that a spec is duplicated (since multiple specs can reference the same compiler)
* 'spack compilers' is now also broken down into sections by os and target
* Added tests for new compiler methods
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