fix: _findMissingEdges missing dependency due to inconsistent path separators

[salvadorj]

When installing a package with bundled dependencies with npm it can end up missing some dependencies that were bundled and attempt to fetch from the remote registry (see npm/cli#2757). This issue only happens on Windows and it's caused by _findMissingEdges missing one of the bundled dependencies. The reason it only happens on Windows is because of this lookup in this[_cache] with a path that contains mixed path separators:

arborist/lib/arborist/load-actual.js

Lines 433 to 435 in 02f295f

	const depPath = `${p}/node_modules/${name}`
	const cached = this[_cache].get(depPath)
	if (!cached || cached.dummy) {

On macOS we get a result back and avoid creating a new node. On Windows we get no result since the key in the cache is using backslashes but the lookup key has both backslashes and forward slashes. This results in a new node being created with the path of an existing node which in turn removes the existing node and its children from the tree. The children are however not loaded from the file system again because of this check:

arborist/lib/arborist/load-actual.js

Lines 353 to 357 in 02f295f

	// impossible except in pathological ELOOP cases
	/* istanbul ignore if */
	if (did.has(node.realpath)) {
	return Promise.resolve(node)
	}

The end result is an unmet dependency and npm attempting to fetch it from the registry instead of using the bundled dependency.

I've changed the cache key by normalizing the path to avoid replacing existing nodes. I've setup a minimal test case where the difference in behavior (with and without normalization) shows up as an error on the outgoing edge of a node.

References

Fixes npm/cli#2757

[fritzy]

@salvadorj This repo is being archived as arborist is now maintained as a workspace in the CLI. I recreated this PR as npm/cli#4261, and the commits maintain you as the author.