Fully genericize install command

[michaeltlombardi]

The install command is extremely similar in pct and prm - the primary differences are in language about the type of package being installed. If install is fully genericized, it can be imported and used in prm without maintaining near-identical code in two repositories and re-used in any other projects.

Acceptance Criteria

• The InstallCommand struct has a new property, ProjectType which takes a string
• The InstallCommand struct renames TemplatePkgPath to InstalledPkgPath
• The instantiation of the install command in main passes the ProjectType as template and updates from TemplatePkgPath to InstalledPkgPath.
• All references to template are genericized, using the ProjectType where sensible (including command help, flag names/descriptions, errors, informational messages, and comments) instead of hard-coding template
• Variables referencing template are updated to package
• Tests updated as necessary