fix(builtin): handle external repository file paths in js_library strip_prefix check

[gregmagolan]

Failure without the fix in js_library.bzl:

ERROR: /private/var/tmp/_bazel_gmagolan/e172f8159a8fef9694f6bd1b99308c59/external/npm/BUILD.bazel:57066:11: in _js_library rule @npm//:testy_copy_js_library: 
Traceback (most recent call last):
	File "/Users/gmagolan/oss/rules_nodejs/internal/js_library/js_library.bzl", line 243, column 30, in _impl
		path = _link_path(ctx, all_files),
	File "/Users/gmagolan/oss/rules_nodejs/internal/js_library/js_library.bzl", line 120, column 17, in _link_path
		fail("js_library %s strip_prefix path does not contain any of the provided sources" % ctx.label)
Error in fail: js_library @npm//:testy_copy_js_library strip_prefix path does not contain any of the provided sources
ERROR: Analysis of target '//internal/js_library/test/strip_prefix:external_test' failed; build aborted: Analysis of target '@npm//:testy_copy_js_library' failed