Distinguish real paths from virtual paths in Filesystem

[DanRStevens]

The Filesystem API has mixed meaning for path parameters, where some paths are real filesystem paths such as the mount calls, while others represent Filesystem provided virtual paths. Perhaps we should use separate types to distinguish these.

It would be possible to wrap the current path data types in new struct data types to distinguish them. Or possibly we want to make the internal path datatype an implementation detail.

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Including the <filesystem> header from Filesystem.h adds significantly to the build time when including Filesystem.h. Perhaps we can use custom struct types for the two types of path parameters, and not expose the internal workings of how they are handled. That could allow us to remove the <filesystem> include from Filesystem.h, and instead only include <filesystem> from the implementation file.

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Related:

• Issue Improve build times OutpostUniverse/OPHD#1573
• Issue Use std::filesystem::path instead of std::string for filepath arguments #973
• PR Use std::filesystem::path in Filesystem #1078