Use namespace prefixes in find_scale()/validate_guide()

[teunbrand]

This PR aims to fix #4705 and replaces #4799.

Essentially it allows you to specifiy a scale/guide using the pkgname::-prefix.
By declaring the namespace we only need to search one namespace and we can avoid searching in all loaded namespaces.
Example guides:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

x <- validate_guide("legendry::colring")
is_guide(x)
#> [1] TRUE

x <- validate_guide("ggraph::edge_direction")
is_guide(x)
#> [1] TRUE

Example scale:

registerS3method(
  "scale_type", "pillar_num", 
  method = function(x) c("pillar::num")
)

is_attached("pillar")
#> [1] FALSE

# Note the class name ends with 'Num'
find_scale("x", pillar::num(1))
#> <ScaleContinuousPositionNum>
#>  Range:  
#>  Limits:    0 --    1

^{Created on 2025-11-17 with reprex v2.1.1}

Practically, packages that implement this may want to declare both the unprefixed and prefixed versions while transitioning.
For example, the definition of the pillar method (here) may look like this:

scale_type.pillar_num <- function(x) c("num", "pillar::num", "continuous")`

[teunbrand]

Instead of adopting earlier advice on using external packages to test this, I'm now using a mocked asNamespace() for testing to return an environment of my choosing.