Disambiguate process buffer names for same-named projects

[bbatsov]

• When two projects share the same name (e.g. two clones of "foo", or two dirs both named "src"), shell/eshell/vterm buffers would collide because projectile-generate-process-name used only the project name
• Now detects when a buffer with the same name already exists for a different project root and falls back to using the abbreviated project path in the buffer name

Fixes #1815
Fixes #1715