This commit adds support for loading import maps from URLs,
both remote and local.
This feature is supported in CLI flag as well as in runtime
compiler API.
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.
