Modules determine their own compatibility with the library

A module is built against the system-installed libdpm-core.so and is
responsible for being correct against it. Where it needs to act on the
version it is running under, dpm_core_version() reports that and the
module decides for itself.

dpm_module_core_min() is removed. It was a declaration handed to the
library to enforce on the module's behalf, and enforcement of that kind
belongs nowhere in a library that routes and hosts. The contract is now
three reserved symbols and load validation is two steps: the reserved
symbols resolve, and the version and description probes return
well-formed values.

compare_versions had no remaining caller and is removed; parse_version
stays for the well-formedness probe. The core_too_new fixture went with
the handshake it existed to exercise.

CMakeLists.txt

@@ -82,7 +82,7 @@ set_target_properties(info PROPERTIES
 add_dependencies(dpm info)
 
 # ---------------------------------------------------------------------
-# Test fixture modules: one known-good stub, three deliberately broken
+# Test fixture modules: one known-good stub, two deliberately broken
 # ---------------------------------------------------------------------
 enable_testing()
 
@@ -95,7 +95,7 @@ file(WRITE ${CMAKE_BINARY_DIR}/DartConfiguration.tcl
 
 set(FIXTURE_MODULE_DIR ${CMAKE_BINARY_DIR}/tests/fixtures)
 
-foreach(fixture good missing_symbols core_too_new bad_version)
+foreach(fixture good missing_symbols bad_version)
     add_library(fixture_${fixture} MODULE tests/fixtures/src/${fixture}.cpp)
     set_target_properties(fixture_${fixture} PROPERTIES
             PREFIX ""
@@ -123,7 +123,7 @@ set_target_properties(test_core PROPERTIES
         BUILD_RPATH "$ORIGIN/../lib"
 )
 
-foreach(fixture good missing_symbols core_too_new bad_version)
+foreach(fixture good missing_symbols bad_version)
     add_dependencies(test_core fixture_${fixture})
 endforeach()