racket/sandbox: allow read of pkg- and config-table files
`find-collects-dir' and `find-config-dir' as built-in, cache values

The computations already existed in the built-in code, so moving
the functions reduces code duplication. Caching the values will
save a little time, but mostly it will avoid sandbox interactions
with the task of locating the main "collects" and "etc" directories.
Revert "Revert "make user PLaneT install path use installation name instead of version""

This reverts commit de230bc1efcf02e29827241bdb53fc571e7072e8.

Now that path computatons are built in, and now that the sandbox allows
reading the configuration file, using the installation name doesn't
break with sandboxing.