The restriction of the `apps._app_name_.command` property is too strict

[林博仁(Buo-ren, Lin)]

I'm migrating a snap with a legacy snapcraft syntax to the new one with bases, however the previous command used to launch the application:

```yaml
    command: >
      bin/zenity
        --info
        --ok-label='Got it'
        --title='My Awesome App'
        --text='This is a demonstrative application for Snapcrafters Template Plus.\nIt indicates that the snapped application has been launched properly.'
        --window-icon="${SNAP}"/snap/gui/my-awesome-app.png
        --width=600
```

no longer work and causes the following Snapcraft error message:

```
Failed to generate snap metadata: The specified command 'bin/zenity\n --info\n --ok-label=\'Got it\'\n --title=\'My Awesome App\'\n --text=\'This is a demonstrative application for Snapcrafters Template Plus.\\nIt indicates that the snapped application has been launched properly.\'\n --window-icon="${SNAP}"/snap/gui/my-awesome-app.png\n --width=600\n' defined in the app 'my-app-name' does not match the pattern expected by snapd.
The command must consist only of alphanumeric characters, spaces, and the following special characters: / . _ # : $ -
```

although this problem can be easily workarounded by using a launcher we probably should reconsider whether we should impose a restriction to this property as it can be any arbitrary string.

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

I encountered this problem again with `command: bin/guvcview --audio=pulse` (the `=` character seems to be the trigger)

[Sergio Schvezov (sergiusens)]

We are following the snapd restrictions now for command, I will add a snapd task to sort this.

[John Lenton (chipaca)]

I agree we want to lift this restriction.

I don't know when we'll find the time to do it (we need to make sure we're not using the fact it's restricted anywhere -- and we probably are).

[Sergio Schvezov (sergiusens)]

Once snapd fixes the issue, snapcraft work can being to make sure we have end to end integration. Removing from the milestone until then.

[Ian Johnson (anonymouse67)]

It looks like the characters being requested here are probably:

- =
- "
- '
- {
- }

In addition, I filed 1827392 against snapd for = and :, so we would have the additional character of

- :

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

I encountered this bug again:

```output
+ snapcraftctl prime
The interpreter 'python3' for 'bin/git-filter-repo' was resolved to '/usr/bin/python3'.
The command 'bin/git-filter-repo' has been changed to '/usr/bin/python3 $SNAP/bin/git-filter-repo' to safely account for the interpreter.
Failed to generate snap metadata: The specified command '/usr/bin/python3 $SNAP/bin/git-filter-repo' defined in the app 'git-filter-repo' does not match the pattern expected by snapd.
The command must consist only of alphanumeric characters, spaces, and the following special characters: / . _ # : $ -
ubuntu #
```

However, this time I failed to see any invalid characters in the `/usr/bin/python3 $SNAP/bin/git-filter-repo` which only contains alpha-numerics, space, dollar sign, and slashes.

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

In the following build:

https://github.com/brlin-tw/snapcrafters-template-plus/actions/runs/3211397149/jobs/5249538457#step:3:1190

```
The interpreter 'bash' for 'bin/my-awesome-app' was resolved to '/bin/bash'.
The command 'bin/my-awesome-app' has been changed to '/bin/bash $SNAP/bin/my-awesome-app' to safely account for the interpreter.
Failed to generate snap metadata: The specified command '/bin/bash $SNAP/bin/my-awesome-app' defined in the app 'my-awesome-app' does not match the pattern expected by snapd.
The command must consist only of alphanumeric characters, spaces, and the following special characters: / . _ # : $ -
```

The command that triggers the error is "/bin/bash $SNAP/bin/my-awesome-app", which doesn't have any of the offending characters at all. There is a problem with the logic determining whether the command is acceptable.

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

Forgot to put on the source that renders the result:

Source: https://github.com/brlin-tw/snapcrafters-template-plus/tree/eff7426446cb7649460efa3c1beab8e70190d1ee