scmake should write a session info file

automatically writing out the session info would help with general reproducibility. would also help clear up any mysterious changes to templated remake files generated by scipiper, if scipiper's template for those files changed.

the contents written out could be a devtools::session_info output or something restricted to just exactly those packages declared in the project's remake files.

should the session info be git committed or ignored? depends on objectives and use case.
* if committed, it could be used as a tool to encourage all collaborators to get on the same page (because they'd see git changes if their session info different from another person's).
* if committed, could be a pain if collaborators don't care about being synced in their package versions
* if committed, a partial build of a project in a fresh session might lead to the loading of fewer packages, such that the session info would appear to be different even if you hadn't installed or updated any packages. would be great if somehow the set of packages written out could be the same every time.

Start by examining the scmake entry point and how project remake files declare packages. The issue does not name files or tests and leaves the session-info contents, package set, and git commit policy unresolved; done requires an agreed reproducible output and behavior for partial builds.

Written by the indexing model from the issue text.