Supports package names that aren't all lowercase.
This stores the package with a leading underscore (since that's not
allowed in npm's registry and no package exists with a leading
underscore) then base32 encoded (A-Z0-9) so it can be lowercased and
avoid collisions.
Global cache dir:
```
$DENO_DIR/npm/registry.npmjs.org/_{base32_encode(package_name).to_lowercase()}/{version}
```
node_modules dir `.deno` folder:
```
node_modules/.deno/_{base32_encode(package_name).to_lowercase()}@{version}/node_modules/<package-name>
```
Within node_modules folder:
```
node_modules/<package-name>
```
So, direct childs of the node_modules folder can have collisions between
packages like `JSON` vs `json`, but this is already something npm itself
doesn't handle well. Plus, Deno doesn't actually ever resolve to the
`node_modules/<package-name>` folder, but just has that for
compatibility. Additionally, packages in the `.deno` dir could have
collissions if they have multiple dependencies that only differ in
casing or a dependency that has different casing, but if someone is
doing that then they're already going to have trouble with npm and they
are asking for trouble in general.
