Call BUILD_POST_INSTALL plugins after module POST_INSTALL
Cascade the proper error code after plugin_call has finished
Added SOURCEx_UPSTREAM to rename downloaded files.

SOURCEx will specify the name as stored in SOURCE_CACHE. The optional
SOURCEx_UPSTREAM specifies the real name of the download file wich will
be renamed during download. SOURCEx_UPSTREAM will not be supported by
all download plugins and might have a different meaning for others. For
example when the SOURCEx_URL is a repository and it support branches the
SOURCEx_UPSTREAM specifies the branch name.
Autogen SOURCES from SOURCE_UPSTREAM if not specified.

This also removes support for holes in SOURCEx. All modules should start
SOURCE and continue with SOURCE2, SOURCE3, etc.