E3-752: Fetch dependencies recursively

This uses some general make tools, most notably "while" and "select" which come from https://github.com/markpiffer/gmtt.git and which allow us to recursively parse the .dep files in the given e3 installation. Broadly speaking, we start with all variables of the form %_DEP_VERSION to determine the first line of dependent modules, and then we ask them (using fetch_deps) what they depend on.

configure/E3/CONFIG_EXPORT

@@ -13,7 +13,7 @@ EXPORT_VARS+=E3_SITEMODS_PATH
 EXPORT_VARS+=$(filter E3_REQUIRE_%,$(.VARIABLES))
 
 EXPORT_VARS+=QUIET
-EXPORT_VARS+=$(filter %_DEP_VERSION,$(.VARIABLES))
+EXPORT_VARS+=$(foreach v,$(.VARIABLES),$(if $(findstring _DEP_VERSION,$v),$v))
 EXPORT_VARS+=$(filter WITH_%,$(.VARIABLES))
 EXPORT_VARS+=$(filter %_EXTERNAL,$(.VARIABLES))