fix: Avoid expanding symlinks in relative path candidates

fix: Avoid expanding symlinks in relative path candidates

Commit 8223ed38abd9cf75ab75cb19dc2081c44bf568d0 changed
make_relative_path to also try matching real_path(path) against the
apparent/actual CWD, assuming that an absolute path p could be
substituted with real_path(p).

This assumption is incorrect for source files: if the source file
includes a file via a relative path, expanding symlinks may cause the
include file to be searched in the wrong directory.

Fix by reverting that change. The new test case "Symlink to source file,
longer symlink path" fails without this fix.

Note: this effectively reverts #724.

Fixes #1620.