configs/configload and internal/initwd both had a copyDir function that
would fail if the source directory contained a symlinked directory,
because the os.FileMode.IsDir() returns false for symlinks.
This PR adds a check for a symlink and copies that symlink in the
target directory. It handles symlinks for both files and directories
(with included tests).
Fixes#20539
Unlike the old installer in config/module, this uses new-style
installation directories that include the static module path so that paths
we show in diagnostics will be more meaningful to the user.
As before, we retrieve the entire "package" associated with the given
source string, rather than any given subdirectory directly, because the
retrieved module may contain ../ references into parent directories which
must be resolvable after extraction.
Kristin Laemmert