fix(npm): support non-all lowercase package names (#16669)

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.

cli/tests/testdata/npm/registry/@denotest/CAPITALS/1.0.0/index.js

@@ -0,0 +1 @@
+module.exports = 5;

cli/tests/testdata/npm/registry/@denotest/CAPITALS/1.0.0/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "@denotest/CAPITALS",
+  "version": "1.0.0"
+}

cli/tests/testdata/npm/registry/@denotest/MixedCase/1.0.0/index.js

@@ -0,0 +1,2 @@
+const value = require("@denotest/CAPITALS");
+module.exports = value;

cli/tests/testdata/npm/registry/@denotest/MixedCase/1.0.0/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "@denotest/MixedCase",
+  "version": "1.0.0",
+  "dependencies": {
+    "@denotest/CAPITALS": "^1"
+  }
+}