rename downloaded files when they do not mirror convention

[aronatkins]

fixes #731

[toph-allen]

The change looks good. Does it need a test?

[aronatkins]

The change looks good. Does it need a test?

Will exercise outside this PR.

Steps to recreate the problem:

rstudioapi::openProject("~/Desktop/r-universe-test", newSession = TRUE)

# Within the new RStudio session
writeLines(c("library(xgboost)"), "script.R")
options(
  repos = c(
    DMLC = "https://dmlc.r-universe.dev",
    CRAN = "https://cran.rstudio.com/"
  )
)
packrat::.snapshotImpl(
  project = ".",
  snapshot.sources = TRUE,
  fallback.ok = TRUE,
  verbose = TRUE,
  implicit.packrat.dependency = FALSE
)

Prior to the fix, this produces:

Adding these packages to packrat:
               _        
    Matrix       1.6-5  
    data.table   1.17.4 
    jsonlite     2.0.0  
    lattice      0.22-7 
    xgboost      3.0.2.1

Fetching sources for Matrix (1.6-5) ... OK (CRAN archived)
Fetching sources for data.table (1.17.4) ... OK (CRAN current)
Fetching sources for jsonlite (2.0.0) ... OK (CRAN current)
Fetching sources for lattice (0.22-7) ... OK (CRAN current)
Fetching sources for xgboost (3.0.2.1) ... FAILED
Error in snapshotSources(project, activeRepos(project), allRecordsFlat) : 
  Errors occurred when fetching source files:
Error in getSourceForPkgRecord(pkgRecord, sourceDir, availablePkgs, repos) : 
  Could not find sources for xgboost (3.0.2.1).

After the fix:

Adding these packages to packrat:
               _        
    Matrix       1.6-5  
    data.table   1.17.4 
    jsonlite     2.0.0  
    lattice      0.22-7 
    xgboost      3.0.2.1

Fetching sources for Matrix (1.6-5) ... OK (CRAN archived)
Fetching sources for data.table (1.17.4) ... OK (CRAN current)
Fetching sources for jsonlite (2.0.0) ... OK (CRAN current)
Fetching sources for lattice (0.22-7) ... OK (CRAN current)
Fetching sources for xgboost (3.0.2.1) ... OK (https://dmlc.r-universe.dev current)
Snapshot written to '/Users/aron/Desktop/r-universe-test/packrat/packrat.lock'