ExpandSubdirGlobs handles a subdir string that might contain glob syntax, turning it into a concrete subdirectory path by referring to the actual files on disk in the given directory which we assume contains the content of whichever package this is a subdirectory glob for.

NormalizePackageAddress uses the go-getter "detector" functionality in order to turn a user-supplied source address into a normalized address which always includes a prefix naming a protocol to fetch with and may also include a transformed/normalized version of the protocol-specific source address included afterward.

SplitPackageSubdir detects whether the given address string has a subdirectory portion, and if so returns a non-empty subDir string along with the trimmed package address.

MaybeRelativePathErr is the error type returned by NormalizePackageAddress if the source address looks like it might be intended to be a relative filesystem path but without the required "./" or "../" prefix.

PackageFetcher is a low-level utility for fetching remote module packages into local filesystem directories in preparation for use by higher-level module installer functionality implemented elsewhere.