tcl9: lib folder not in auto_path

Dear TCL team,
I have build and installed TCL 9.0.0 using the nmake with 32 bit MS-VS 2015 to folder c:\test\tcl9.0.0\tcl900
The lib folder containes the bundled packages, like:
c:\test\tcl9.0.0\tcl900\lib\tdbc
% package require TDBC
1.1.9
% package files tdbc
C:/test/tcl9.0.0/tcl900/lib/tdbc1.1.9/pkgIndex.tcl C:/test/tcl9.0.0/tcl900/lib/tdbc1.1.9/tdbc.tcl
Then, I have installed TCLLIB 2.0 to its lib folder:
c:\test\tcl9.0.0\tcl900\lib
(remark that the installer has proposed to embed it into the zip. I have changed this).
For example, the package log is in folder
c:\test\tcl9.0.0\tcl900\lib\log
But:
% package require log
-> no such package
% set auto_path
//zipfs:/lib/tcl/tcl_library //zipfs:/lib/tcl C:/test/tcl9.0.0/tcl900/lib //zipfs:/lib/tk/tk_library //zipfs:/lib/tk/tk_library/ttk
-> the lib folder is not in the auto path
% lappend auto_path {C:\test\tcl9.0.0\tcl900\lib}
% package require log
1.5
How is tdbc loaded, if not in the auto path?
Is it good practice to add the external lib folder to the auto path?
Thank you all,
Harald