micropython/mip: Allow relative URLs in package.json files.

[glenn20]

This PR allows URLs in package.json files to be specified relative to the directory containing the package.json file. This mirrors the functionality added to mpremote in micropython:#12477.

This simplifies use of mip packages from forked or copied repos.

Example package.json:

{
    "urls": [
        ["ota/rollback.py", "rollback.py"],
        ["ota/status.py", "status.py"],
        ["ota/update.py", "update.py"],
        ["ota/blockdev_writer.py", "blockdev_writer.py"]
    ],
    "deps": [
    ],
    "version": "0.2"
}

[glenn20]

Basic functionality can be tested with:

mip.install("github:glenn20/micropython-esp32-ota/mip/ota/package-relative-url.json")

[dpgeorge]

Thank you!

I tested it with a test package hosted at https://dpgeorge.net/resources/mip_test and it works well.

[dpgeorge]

Side note: I think we should add support in MicroPython core to pass a tuple of strings to str.startswith/str.endswith.

[glenn20]

Side note: I think we should add support in MicroPython core to pass a tuple of strings to str.startswith/str.endswith.

@dpgeorge I've just pushed #16812 to provide that (and also support for start and end args).