`install_version` allows non-CRAN-like repositories

The existing `install_version` function assumes a CRAN-like repository structure, which is hard to emulate for private repositories. The new version adds an `archive_structure` parameter which allows the user to install a specific version of the package if the package repository has a flat archive structure (i.e. all versions of every package are located in the same directory).

```R
archive_structure = TRUE # CRAN-like package repository (with archive directory)
archive_structure = FALSE # non-CRAN-like repository
```