Make symlink_forest.go prefer generated files.

Now, if the same file exists in the generated tree and the source tree,
it symlinks in the generated file instead of failing outright.

Drive-by fix: print errors for all conflicts instead of bailing out on
the first one.

Test: Presubmits (including the two new tests)
Change-Id: Ifb5b3fc89b5454d231966bfa4e61c22cd69834f3