bug#52684: [BUG] Multiple Packages Failing to Build

[Christopher Rodriguez <yewscion@gmail.com>]

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So, I took some time to do some digging this morning, and I now have a 
few results and a few more questions.

First, I tried `guix shell --pure python beets beets-bandcamp` to ensure 
that the plugin would be detected once `GUIX_PYTHONPATH` was set as You 
had mentioned. That did work, though the environment did not have 
`python-isodate`, which the plugin complained was missing. If this is 
the case, should python-isodate then be a progagated input, since it is 
apparently required at runtime? Or is there a difference w/r/t the
`--pure` environment that won't be present on an actual install?

Second, I ran `cat $(which beet)` on my currently installed `beets` 
package for the `PYTHON_PATH` that is currently being set. It has a lot 
of entries, all like the following: 
`/gnu/store/8df74df68i14lfjsny07x7cq6ffn0fs5-beets-1.5.0/lib/python3.8/site-packages:/gnu/store/nc3rprg62sigx8s9ps02wb8zbaz8qzl3-python-flask-1.1.2/lib/python3.8/site-packages`. 
I'm currently diving each of these packages to see how I might add code 
to set `beets-bandcamp` to add to this list, though I wonder if that is 
actually possible, since the plugin will usually be installed after the 
root program. Is this path pulled from a global location, or is it 
determined at install time?

Third, I did try pulling

```scheme
(native-search-paths
   (list (guix-pythonpath-search-path)))
```

into the `beets` definition, but that brought a *lot* of other 
dependencies in. Same with using

```scheme
(native-inputs 
 

   `(("sitecustomize.py" ,(local-file (search-auxiliary-file 
 

        "python/sitecustomize.py")))))
```

Sorry if I am missing something obvious, here. I will keep looking, just 
wanted to log what I've tried so far.

Once I figure out how search paths actually work inside of `guix`, I'm 
sure the solution will be fairly simple. But I haven't quite gotten 
there yet.

It seems like the solution has to do with `wrap-package`, but looking at 
the `beets` package definition, I only see the following lines related 
to wrapping the binary:

```scheme
          ;; Wrap the executable, so it can find python-gi (aka 
 

          ;; pygobject) and gstreamer plugins. 
 

          (add-after 'wrap 'wrap-typelib
            (lambda* (#:key outputs #:allow-other-keys)
              (let ((prog (string-append (assoc-ref outputs "out")
                                         "/bin/beet"))
                    (plugins (getenv "GST_PLUGIN_SYSTEM_PATH"))
                    (types (getenv "GI_TYPELIB_PATH")))
                (wrap-program prog
                  `("GST_PLUGIN_SYSTEM_PATH" ":" prefix (,plugins))
                  `("GI_TYPELIB_PATH" ":" prefix (,types)))
                #t))))))
```
It doesn't seem like PYTHONPATH is being interacted with here. Is that 
something that might be defined elsewhere? I'm tempted to merge in the 
following, which I pulled from another package (solfege), to explicitly 
set PYTHONPATH… But I don't want to change something if it is set elsewhere.


```scheme
          (add-after 'install 'wrap-program
            (lambda* (#:key inputs outputs #:allow-other-keys)
              ;; Make sure 'solfege' runs with the correct PYTHONPATH. 
 

              (let* ((out (assoc-ref outputs "out"))
                     (path (getenv "GUIX_PYTHONPATH")))
                (wrap-program (string-append out "/bin/solfege")
                  `("GUIX_PYTHONPATH" ":" prefix (,path))))
              #t)))))
```

Any input or clarification will be appreciated! I'm learning a lot as I 
(slowly) work through this! Thank You for Your time!

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