A line of the format
   package provide Tls 1.7.19
is desirable somewhere in tls.tcl, such that package requests with
   package require Tls
from another program get a positive response that the package is present.

The Tcl API call Tcl_PkgProvide() is called from Tls_Init() as part of loading
the TLS extension.  There wouldn't be any advantage to adding it to tls.tcl.

What are you trying to accomplish ?

The "package provide" is a standard feature of RPM build systems; it does not mean that at the time of calling "package provide" some Tls_init() has already happened or is available. We wish to cover situations inside a RPM build to *request* installation of that Tcl package, before running anything *after* the package might have been installed.

The standard method for performing a package provide is to include it in the
source code file that implements the package as part of the initialization
function. This ensures the package is only available if it has been
successfully loaded and initialized.