core: clearly define when module lookup is path-based vs URL-based

The rules are now as follows:

* In `import` statements, as mandated by the WHATWG specification,
  the import specifier is always treated as a URL.
  If it is a relative URL, it must start with either / or ./ or ../

* A script name passed to deno as a command line argument may be either
  an absolute URL or a local path.
  - If the name starts with a valid URI scheme followed by a colon, e.g.
    'http:', 'https:', 'file:', 'foo+bar:', it always interpreted as a
    URL (even if Deno doesn't support the indicated protocol).
  - Otherwise, the script name is interpreted as a local path. The local
    path may be relative, and operating system semantics determine how
    it is resolved. Prefixing a relative path with ./ is not required.

cli/deno_error.rs

@@ -110,7 +110,8 @@ impl DenoError {
         use ModuleResolutionError::*;
         match err {
           InvalidUrl(err) | InvalidBaseUrl(err) => Self::url_error_kind(err),
-          ImportPathPrefixMissing => ErrorKind::ImportPathPrefixMissing,
+          InvalidPath => ErrorKind::InvalidPath,
+          ImportPrefixMissing => ErrorKind::ImportPrefixMissing,
         }
       }
       Repr::Diagnostic(ref _err) => ErrorKind::Diagnostic,