plugin download/execution improvements

[j-lanson]

Resolves #706 . Resolves #707 .

Some quality of life changes for plugins.

• Add the plugin's cache path to the plugin's process PATH to help plugin process find artifacts when running
• Add force: bool to plugin downloading function, and use existing plugin with right version in cache if exists and force=false instead of pulling and decompressing every time
• Change plugin bundle extraction to only copy up artifacts from a subdir if it matches name of the bundle minus the compression extension. Previous impl could accidently copy unrelated files if passed extract_dir contained other subdirs already.

[patrickjcasey]

Looks good and functions as expected on my machine!