CanonPath, SourcePath: Change operator + to /

[edolstra]

Motivation

This is less confusing and makes it more similar to std::filesystem::path.

Context

Priorities and Process

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[Ericson2314]

std::filesystem::path also works this way, so if we use that more this will keep things consistent.

[roberth]

Hey, I thought of this at some point

• Overload / to mean path append; alternative to path + "/" + "foo" footgun #7301 (comment)

The issue is mainly about doing this in the expression language though, but this is great!