Should capsule transform RSPM repo urls

[MilesMcBain]

When writing a lockfile, currently {capsule} diverges from {renv} with respect to RSPM repo URLS.

On a system with:

> options('repos')
$repos
                                                           RSPM 
"https://packagemanager.rstudio.com/all/__linux__/focal/latest" 
                                                           CRAN 
                                     "https://cran.rstudio.com"

{capsule} will write:

    "Repositories": [
      {
        "Name": "RSPM",
        "URL": "https://packagemanager.rstudio.com/all/__linux__/focal/latest"
      },
      {
        "Name": "CRAN",
        "URL": "https://cran.rstudio.com"
      }
    ]

while {renv} writes:

    "Repositories": [
      {
        "Name": "RSPM",
        "URL": "https://packagemanager.rstudio.com/all/latest"
      },
      {
        "Name": "CRAN",
        "URL": "https://cran.rstudio.com"
      }
    ]

Here {renv} has transformed the platform specific binary url for RSPM into a general source package url. However when restoring from the lockfile {renv} will attempt to transform the source url into a platform specific binary one.

I am surprised this hasn't caused issues at work with collaborators working on windows and linux with different repository URLs, so it seems like {renv} falls back to the local system's repo urls if it can't use the lockfile ones. Need to confirm this, but if so, I see no reason to mirror this complexity.

[MilesMcBain]

I can't see a fallback in {renv}.

So I think a better way of handling this is a new option for resolving repos on restore. Renv uses the lockfile repos with the option to override.

I'll use that override to provide a 'merge' option for repo resolution, which is default for capsule.

Mege works like:

1. Where conflicting entries exist for a major repo (CRAN, bioc, rspm) prefer the user's configured repo.
2. If a major repo exists in one but not the other, bring it in.
3. Major repos have a defined order: bioc, rspm, CRAN.
4. All other unique repos are brought in, with a message for those coming from the lockfile. - where to place these in the order will be tricky. Probably they should come after the majors, unless the user has explicitly confuguired them otherwise.