[FR]: Better support for external directory files in copy_to_directory

### What is the current behavior?

Here's a simple use case where I would like to copy an external folder from our `node_modules`:
`/dist/fonts` => `/material-design-icons-iconfont`.

Currently, I seem to be unable to figure out which combinations of props will give me the desired result. Let's say I use the package `material-design-icons-iconfont@6.7.0` the prefix would need to be hardcoded:

Currently:
```py
copy_to_directory(
name = "material-design-icons",
srcs = [
"//:node_modules/material-design-icons-iconfont/dir",
],
out = "material-design-icons-iconfont",
replace_prefixes = {
# copies an empty `node_modules` folder, not sure where this comes from
"node_modules": "",
# would expect to use $(location ...) here
"node_modules/.aspect_rules_js/material-design-icons-iconfont@6.7.0/node_modules/material-design-icons-iconfont/dist/fonts": "",
},
)
```

Expected:
```py
copy_to_directory(
name = "material-design-icons",
srcs = [
"//:node_modules/material-design-icons-iconfont/dir",
],
out = "material-design-icons-iconfont",
replace_prefixes = {
"$(location //:node_modules/material-design-icons-iconfont/dir)/dist/fonts": "",
},
)

# or

copy_to_directory(
name = "material-design-icons",
srcs = [
"//:node_modules/material-design-icons-iconfont/dir",
],
out = "material-design-icons-iconfont",
root_paths = [
"$(location //:node_modules/material-design-icons-iconfont/dir)",
],
replace_prefixes = {
"dist/fonts": "",
},
)
```

### Describe the feature

I would like to avoid having to compute the path to strip or hardcoding the value. Possibly there is a better way to approach this copy operation? I have tried using `root_paths` as well with no success, either way, it would be great to have a convenient way of performing this operation.

### Fund our work

- [ ] Sponsor our open source work by donating a [feature bounty](https://opencollective.com/aspect-build/)

Start at the copy_to_directory entry point and trace how root_paths and replace_prefixes handle external directories and location-expanded paths. Reproduce the material-design-icons example, then verify that the desired directory can be copied using a location-derived path without hardcoding the node_modules prefix.

Written by the indexing model from the issue text.