Add DOWNLOAD-SHELF restart for LOAD-SHELF, which allows interactive user to download shelf when some ASDF system is not found.

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Use EXTERNAL-PROGRAM instead of TRIVIAL-SHELL; don't do symlinks (Librarian should work natively on Windows now).

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System-definition-pathnames returns the path of the asd file, even if it's a
symlink. Changing this function to `component-pathname' resolves this problem.

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darcs-hash:220e2f3f4c8443634f1a8075c1e239ae6807646c

Remove separate shelf systems dir; rearrange shelf directory into site/ subdir with library and systems/ subdir with .asd file symlinks.

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