I'm trying to install Python with tkinter support using the Spack package manager. Spack adds the following flags to configure during install:
```
'--with-tcltk-libs=-L/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/lib -ltcl8.6 -L/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tk-8.6.11-ydmhrbboheucxsuhrnyoxqaihgna5dfe/lib -ltk8.6'
```
It also sets the following environment variables:
```
TCLLIBPATH='/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/lib/tcl8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tk-8.6.11-ydmhrbboheucxsuhrnyoxqaihgna5dfe/lib/tcl8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tk-8.6.11-ydmhrbboheucxsuhrnyoxqaihgna5dfe/lib64/tcl8.6'; export TCLLIBPATH
TCL_LIBRARY=/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/lib; export TCL_LIBRARY
```
The install seems to correctly pick up tk/tcl and builds correctly. However, when I try to use tkinter, I see the following run-time error:
```
$ python
Python 3.8.10 (default, May 27 2021, 13:28:01) 
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import tkinter
>>> tkinter._test()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.8.10-fkj5vkn3tpottyv6yqoj5ucz2emstpvo/lib/python3.8/tkinter/__init__.py", line 4557, in _test
    root = Tk()
  File "/Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.8.10-fkj5vkn3tpottyv6yqoj5ucz2emstpvo/lib/python3.8/tkinter/__init__.py", line 2270, in __init__
    self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
_tkinter.TclError: Can't find a usable tk.tcl in the following directories: 
    /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tk-8.6.11-ydmhrbboheucxsuhrnyoxqaihgna5dfe/lib /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/lib/tcl8.6/tk8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/lib/tcl8.6/tk8.6/Resources/Scripts /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tk-8.6.11-ydmhrbboheucxsuhrnyoxqaihgna5dfe/lib/tcl8.6/tk8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tk-8.6.11-ydmhrbboheucxsuhrnyoxqaihgna5dfe/lib/tcl8.6/tk8.6/Resources/Scripts /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tk-8.6.11-ydmhrbboheucxsuhrnyoxqaihgna5dfe/lib64/tcl8.6/tk8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tk-8.6.11-ydmhrbboheucxsuhrnyoxqaihgna5dfe/lib64/tcl8.6/tk8.6/Resources/Scripts /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/lib/tk8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/lib/tk8.6/Resources/Scripts /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/tk8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/tcl-8.6.11-n7nea33urrk25rkoqpsc2tdcgai5u4z2/tk8.6/Resources/Scripts /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.8.10-fkj5vkn3tpottyv6yqoj5ucz2emstpvo/lib/tk8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.8.10-fkj5vkn3tpottyv6yqoj5ucz2emstpvo/lib/tk8.6/Resources/Scripts /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.8.10-fkj5vkn3tpottyv6yqoj5ucz2emstpvo/lib/tk8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/lib/tk8.6 /Users/Adam/spack/opt/spack/darwin-catalina-x86_64/apple-clang-12.0.0/python-3.8.10-fkj5vkn3tpottyv6yqoj5ucz2emstpvo/library

This probably means that tk wasn't installed properly.
```
It seems that tkinter searches for tk.tcl in `<prefix>/lib`, but tk.tcl is actually installed in `<prefix>/lib/tk8.6`. I asked the tk developers, but it looks like `<prefix>/lib/tk8.6` is indeed the correct installation location: https://core.tcl-lang.org/tk/tktview/447bd3e4abe17452d19a80e6840dcc8a2603fcbc

Is there a way to tell tkinter where to find tk.tcl? If not, can we modify the default search path to search in `<prefix>/lib/tk8.6`?

Related to https://github.com/spack/spack/issues/23780