Add --path flag to pack extension package

[robdimsdale]

Description

Add --path to pack extension package, similar to what exists in pack buildpack package.

I would like this for symmetry between extensions and buildpacks. It will streamline scripts/tooling that interact with both options. See e.g. this PR to paketo scripts, where adding pack extension package requires extra boilerplate to work around the lack of --path.

I admit to not knowing too much about the internal differences, so if there's a good reason this flag can't exist I absolutely understand and accept that.

[sshiv5768]

@jjbustamante @robdimsdale I would like to work on this issue.

[natalieparellano]

@sshiv5768 we'd love to have your help here! I've assigned you. Feel free to reach out here or in Slack (#buildpacks-pack in CNCF Slack) if you'd like further guidance.

[sshiv5768]

Thanks for assigning me this issue. @natalieparellano i wasn't able find #buildpacks-pack channel in CNCF slack.

[natalieparellano]

i wasn't able find #buildpacks-pack channel in CNCF slack.

Apologies! It is #buildpacks-pack-cli