RFC: KLibexec: Use the canonical path to the library/application as base

This should make it work in the case of /lib* -> /usr/lib* links. See the added comment for details.

BUG: 452817

This feels less hacky than doing

if(!QFile::exists(libexecPath)) {
    return QStringLiteral("/usr") + libexecPath;
}

but might introduce regressions in cases at other edges and has slightly more overhead. Opinions?

CC @sitter