Two questions on package development

General Usage
5

Yes. Here is the documentation:

https://pkgdocs.julialang.org/v1/api/#Pkg.add

In particular, take a look at PackageSpec which has a path keyword argument.
https://pkgdocs.julialang.org/v1/api/#Pkg.PackageSpec

Let’s say I use PkgTemplates and customize dir to be ~/src so the package lives at ~/src/MyPkg as follows:

julia> using PkgTemplates

julia> Template(interactive=true)("MyPkg")
Template keywords to customize:
[press: d=done, a=all, n=none]
   [ ] user
   [ ] authors
 > [X] dir
   [ ] host
   [ ] julia
   [ ] plugins
Enter value for 'dir' (String, default="~/.julia/dev"): ~/src
[ Info: Running prehooks
[ Info: Running hooks
  Activating project at `~/src/MyPkg`
...

Then I use the Pkg REPL as follows to add the package to a temporary environment:

(@v1.7) pkg> activate --temp
  Activating new project at `/tmp/jl_OOV4GX`

(jl_OOV4GX) pkg> add ~/src/MyPkg
     Cloning git-repo `/home/mkitti/src/MyPkg`
    Updating git-repo `/home/mkitti/src/MyPkg`
   Resolving package versions...
    Updating `/tmp/jl_OOV4GX/Project.toml`
  [6b62e0e2] + MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main`
    Updating `/tmp/jl_OOV4GX/Manifest.toml`
  [6b62e0e2] + MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main`
Precompiling project...
  1 dependency successfully precompiled in 1 seconds

As Chris, mentioned, I can also dev the package:

(jl_OOV4GX) pkg> dev ~/src/MyPkg
   Resolving package versions...
    Updating `/tmp/jl_OOV4GX/Project.toml`
  [6b62e0e2] ~ MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main` ⇒ v0.1.0 `~/src/MyPkg`
    Updating `/tmp/jl_OOV4GX/Manifest.toml`
  [6b62e0e2] ~ MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main` ⇒ v0.1.0 `~/src/MyPkg`

If you prefer to use the Pkg API, this is the equivalent of the above:

julia> using Pkg

julia> Pkg.activate(temp=true)
  Activating new project at `/tmp/jl_xGkOLr`

julia> Pkg.add(path="/home/mkitti/src/MyPkg")
    Updating git-repo `/home/mkitti/src/MyPkg`
   Resolving package versions...
    Updating `/tmp/jl_xGkOLr/Project.toml`
  [6b62e0e2] + MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main`
    Updating `/tmp/jl_xGkOLr/Manifest.toml`
  [6b62e0e2] + MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main`

julia> Pkg.develop(path="/home/mkitti/src/MyPkg")
   Resolving package versions...
    Updating `/tmp/jl_xGkOLr/Project.toml`
  [6b62e0e2] ~ MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main` ⇒ v0.1.0 `~/src/MyPkg`
    Updating `/tmp/jl_xGkOLr/Manifest.toml`
  [6b62e0e2] ~ MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main` ⇒ v0.1.0 `~/src/MyPkg`

If you want to use PackageSpec directly, that is also possible:

julia> using Pkg

julia> mypkg = PackageSpec(path="/home/mkitti/src/MyPkg")
PackageSpec(
  path = /home/mkitti/src/MyPkg
  version = *
)

julia> Pkg.add(mypkg)
    Updating git-repo `/home/mkitti/src/MyPkg`
   Resolving package versions...
    Updating `/tmp/jl_atZNj8/Project.toml`
  [6b62e0e2] + MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main`
    Updating `/tmp/jl_atZNj8/Manifest.toml`
  [6b62e0e2] + MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main`

julia> Pkg.develop(mypkg)
   Resolving package versions...
    Updating `/tmp/jl_atZNj8/Project.toml`
  [6b62e0e2] ~ MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main` ⇒ v0.1.0 `~/src/MyPkg`
    Updating `/tmp/jl_atZNj8/Manifest.toml`
  [6b62e0e2] ~ MyPkg v0.1.0 `/home/mkitti/src/MyPkg#main` ⇒ v0.1.0 `~/src/MyPkg`

Just in case it is confusing, you only have to use either add or develop.