Allow direct mounting directory 'pattern' (plus improve errors)

[rylev]

Fixes #1487 and #1356. This PR does the following two things

Direct mounting directory "patterns"

When the files option is a directory that exists and --direct-mounts is passed, we now allow mapping to happen (previously this was an error). Essentially this makes files = ["some/directory"] equivalent to files = { source = "some/directory", destination = "some/directory" }.

Better error messages

The error message when using a file or glob pattern together with --direct-mounts was the following:

Error: Failed to prepare configuration

Caused by:
    this component cannot be run with `--direct-mount` because it uses file patterns

This was very confusing especially when using a file path and not glob pattern.

The error message now changes to the following in these various situations:

Glob pattern

Error: Failed to prepare configuration

Caused by:
    0: `--direct-mount` flag cannot be used for this spin application
    1: only directories can be directly mounted but attempted to mount the glob pattern 'my-pattern*'

Existing file path

Error: Failed to prepare configuration

Caused by:
    0: `--direct-mount` flag cannot be used for this spin application
    1: only directories can be directly mounted but attempted to mount the file path 'some/file.txt'

Non-existing directory or file path

Error: Failed to prepare configuration

Caused by:
    0: `--direct-mount` flag cannot be used for this spin application
    1: only directories can be directly mounted but attempted to mount the non-existing path 'does/not/exist'

[itowlson]

Thanks for this - love the improvement, just some nits on the implementation.

[itowlson]

This is an extremely complicated clause, especially at the "an else inside a match inside a map" level of indentation - by the time I get to the end of it I have long since forgotten what I'm doing. Consider pulling out a function e.g. direct_mount_pattern_error(...), or even (maybe better) pulling out the whole match arm (starting on line 79) into direct_mount_for_pattern().

Or even pulling out the whole map body...

[rylev]

I pulled out the map body into its own function. I hope that that helps!

[itowlson]

IT HELPS SO MUCH