environment views: better, earlier error on clash (#35541)

Spack generally ignores file-file projection clashes in environment
views, but would eventually error when linking the `.spack` directory
for two specs of the same package.

This leads to obscure errors where users have no clue what the issue is
and how to fix it. On top of that, the error comes very late, since it
happens when the .spack dir contents are linked (which happens after
everything else)

This PR improves that by doing a quick check ahead of time if clashes
are going to be anticipated (by simply checking for clashes in the
projection of each spec's .spack metadir). If there are clashes, a
human-readable error is thrown which shows two of the conflicting specs,
and tells users to user unify:true, view:false, or set up custom
projections.

lib/spack/llnl/util/link_tree.py

@@ -430,6 +430,11 @@ class MergeConflictError(Exception):
     pass
 
 
+class ConflictingSpecsError(MergeConflictError):
+    def __init__(self, spec_1, spec_2):
+        super(MergeConflictError, self).__init__(spec_1, spec_2)
+
+
 class SingleMergeConflictError(MergeConflictError):
     def __init__(self, path):
         super(MergeConflictError, self).__init__("Package merge blocked by file: %s" % path)