Profile resolver: Follow keep instruction for back-matter resources

[galtm]

If a back-matter resource has a keep instruction, the spec
requirement "req-backmatter-keep" says not to replace the
resource unless a later one also has a keep instruction.

Committer Notes

This pull request fixes a bug I noticed while working on #1377. If the merge phase replaces a resource having a child element <prop name="keep" value="always"/> with a resource that does not, then the finish phase cannot follow the specification's requirement "req-backmatter-keep".

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[galtm]

@wendellpiez , @david-waltermire-nist : If the eventual resolution of #1314 requires different logic here, let me know and I can revisit this code.

[wendellpiez]

Looking good thanks @galtm !

[david-waltermire]

LGTM. Thanks!