tcl-tk: add main lib dir to package search path
tcl-tk: add main lib dir to package search path

Currently, `tcl-tk`'s package search path only includes its own directories:
```
$ /usr/local/opt/tcl-tk/bin/tclsh <<<'puts $auto_path'
/usr/local/Cellar/tcl-tk/8.6.12_1/lib/tcl8.6 /usr/local/Cellar/tcl-tk/8.6.12_1/lib /usr/local/opt/tcl-tk/lib

$ ls /usr/local/lib/expect5.45.4/
libexpect5.45.4.dylib  pkgIndex.tcl

$ /usr/local/opt/tcl-tk/bin/tclsh <<<'puts [package require Expect]'
can't find package Expect
```
so packages from other formulae like `expect` can't be loaded, even after they're linked into the main lib dir at install time.

This PR adds the main lib dir to the search path:
```
$ /opt/homebrew/opt/tcl-tk/bin/tclsh <<<'puts $auto_path'
/opt/homebrew/Cellar/tcl-tk/8.6.12_2/lib/tcl8.6 /opt/homebrew/Cellar/tcl-tk/8.6.12_2/lib /opt/homebrew/opt/tcl-tk/lib /opt/homebrew/lib

$ ls /opt/homebrew/lib/expect5.45.4/
libexpect5.45.4.dylib   pkgIndex.tcl

$ /opt/homebrew/opt/tcl-tk/bin/tclsh <<<'puts [package require Expect]'
5.45.4
```
Also added a test to confirm the path, in case upstream breaks this configuration process at some point.

Closes #116615.

Signed-off-by: Jonathan Chang <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@jonathanchang.org>
Signed-off-by: BrewTestBot <8a898ee6867e4f2028e63d2a6319b2224641c06c@users.noreply.github.com>

Homebrew/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core,filcab/homebrew-core