ExternalData: Collapse ../ components in DATA{} paths

Relative path components need to be normalized out even if they appear
in the middle of a caller-supplied string.

Tests/Module/ExternalData/Data2/CMakeLists.txt

@@ -4,6 +4,7 @@ ExternalData_Add_Test(Data2
   NAME Data2Check
   COMMAND ${CMAKE_COMMAND}
     -D Data2=DATA{../Data2.dat}
+    -D Data2b=DATA{${CMAKE_CURRENT_SOURCE_DIR}/../Data2b.dat}
     -D SeriesC=DATA{SeriesC_1_.my.dat,:}
     -P ${CMAKE_CURRENT_SOURCE_DIR}/Data2Check.cmake
   )

Tests/Module/ExternalData/Data2/Data2Check.cmake

@@ -1,6 +1,9 @@
-if(NOT EXISTS "${Data2}")
-  message(SEND_ERROR "Input file:\n  ${Data2}\ndoes not exist!")
-endif()
+foreach(d "${Data2}" "${Data2b}")
+  file(STRINGS "${d}" lines LIMIT_INPUT 1024)
+  if(NOT "x${lines}" STREQUAL "xInput file already transformed.")
+    message(SEND_ERROR "Input file:\n  ${d}\ndoes not have expected content, but [[${lines}]]")
+  endif()
+endforeach()
 foreach(n 1 2 3)
   string(REGEX REPLACE "_1_\\.my\\.dat$" "_${n}_.my.dat" SeriesCFile "${SeriesC}")
   if(NOT EXISTS "${SeriesCFile}")

Tests/Module/ExternalData/Data2b.dat.md5

@@ -0,0 +1 @@
+8c018830e3efa5caf3c7415028335a57