Extensions: Require slashes for local installs

[andreaphylum]

Currently, phylum extension add will install extensions from a local directory.

In the future, we may have a marketplace of extensions and it would make more sense that phylum extension add <extension-name> adds an extension from there.

To ensure that we can distinguish between local and marketplace installations in the future, the phylum extension add should enforce a specific format for local installs.

After discussion, we have decided that a local install will be required to use a slash in the phylum extension add command. For example:

phylum extension add foo          # Install `foo` from marketplace
phylum extension add ./foo        # Install from local directory `foo`
phylum extension add foo/bar      # Install from local directory `foo/bar`
phylum extension add /path/to/foo # Install from local directory `/path/to/foo`
phylum extension add C:\dev\foo   # Install from local directory `C:\dev\foo`

Error message

Since we do not currently support marketplace installs, any phylum extension add command that attempt to install from the marketplace will fail with a descriptive error message:

❗ Error: Attempting to install from marketplace, which is not yet supported. Did you mean `phylum extension add ./foo`

[cd-work]

IIRC your current PR looks for an extension file in the directory, but if we force people to specify the file directly we could determine if the extension is a local file or a remote extension name using the CLI argument alone.

so phylum extension add ./my-plugin/extension.toml versus phylum extension add my-plugin.

[kylewillmon]

so phylum extension add ./my-plugin/extension.toml versus phylum extension add my-plugin.

What would this be using to make the determination? What happens if my-plugin is a valid TOML file in the current directory?

[cd-work]

I would just base it solely on the filename. The existing PR already enforces a specific name for the toml file, so we can use that to determine the format?

[kylewillmon]

After discussion, we have decided to require slashes in the argument for local installs. I have updated the title and description accordingly.