Only need to look up internal exes in `exe_map`

This map, I am told, only contains component ids for the
current package, so there is no point of looking for other
package's exe components in there.

Sent to travis to confirm hypothesis, but amended commit
to elabote this message.

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -1275,10 +1275,9 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
                     (concatMap (elaborateExeSolverId mapDep) external_exe_dep_sids) ++
                 cc_internal_build_tools cc
             compExeDependencyPaths =
-                concatMap (elaborateExePath mapDep)
-                          external_exe_dep_sids ++
+                concatMap (elaborateExePath mapDep) external_exe_dep_sids ++
                 [ path
-                | cid' <- compExeDependencies
+                | cid' <- cc_internal_build_tools cc
                 , Just path <- [Map.lookup cid' exe_map]]
 
             bi = Cabal.componentBuildInfo comp