[BUG][R] use loadNamespace instead of package:pkgName string

[LiNk-NY]

Hi William, @wing328

This change makes use of loadNamespace instead of package:{{packageName}}.
The latter requires that the package be attached to the search path when using
as.environment (this can cause issues when it is not). The current change loads
the namespace before its use.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc: @Ramanth @saigiridhar21

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[wing328]

Looks good to me. If no one has further feedback, I'll merge it on the coming Friday.

[wing328]

@LiNk-NY thanks for the PR, which has been included in the v4.2.2 release: https://twitter.com/oas_generator/status/1201432648544972800