HTTPRepository: improving handling of archives

Avoid hard-coding the archive extension, and ensure the extracted
archive directory is not orphaned on update. Finally, use the
literal filename in the .dirindex when computing the hash, rather
than adding a .tgz extension.

CMakeLists.txt

@@ -286,6 +286,7 @@ else()
     find_package(ZLIB 1.2.4 REQUIRED)
 endif()
 
+find_package(LibLZMA REQUIRED)
 find_package(CURL REQUIRED)
 
 if (SYSTEM_EXPAT)