Dvc: Implement `--no-exec` option for `import-url` command

[anotherbugmaster]

dvc import-url creates new .dvc file, just as dvc run. Sometimes files which would be imported are already present locally and it's quite inconvenient that they should be downloaded again in order to create a pipeline step.

Because of that it would be great to add --no-exec option: we create pipeline step, then use dvc commit to update its md5 with already downloaded file.

[shcheklein]

Context: https://discordapp.com/channels/485586884165107732/485596304961962003/690194007816798243

[Eliska-0111]

taking a look

[jorgeorpinel]

I know this is kind of old! But another alternative is to create the import .dvc file manually. We've documented the spec better now so it should be doable: https://dvc.org/doc/user-guide/dvc-files-and-directories#dvc-files

That's assuming commit will calculate the eTag/MD5/checksum value...

p.s. we should probably have this option in regular import too for the same reasons and consistency (see #4567)

[anotherbugmaster]

@jorgeorpinel good point, that's exactly what I did as a workaround actually :)