Make proposal for names found in top-level scope in a source file

[jonmeow]

The meaning of the top-level scope in a source file was the topic of leads question #1136. The question was resolved and the main decision comment is https://github.com/carbon-language/carbon-lang/issues/1136#issuecomment-1086352687

Because the question was asked independent of a proposal, a standard proposal is still needed for evolution review, essentially translating the issue into a proposed solution with rationale, alternatives considered, etc. This issue is to track the proposal need.

[jonmeow]

Note, #2001 also covers making the package name parameter for import optional. This was brought up due to #2323.

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[josh11b]

#2550 partially covers this.