Disable the merging of license and copyright information from different sources

[linozen]

Currently, if reuse finds copyright/license information both in a ./reuse/dep5 and a license header this information is merged. This is undesirable as file headers should be given clear precedence over some globbing statement in ./reuse/dep5.

Resolving this only partially resolves #70 but it is step in the right direction nonetheless.

[mxmehl]

Closed with the new --json feature in reuse-tool, and the definition in #131