Fix cache/path issue with npm 7

[CL-Jeremy]

Tested on Windows 10 (MSYS2), Ubuntu 18.04 amd64 (VMware Fusion), Fedora 33, Manjaro 18.1, with both npm 6 and npm 7.

The only awkward result was on the Ubuntu VM, where installing Fomantic is blazing fast with npm 6 but takes quite some time (more than 40 seconds) to complete with npm 7, probably due to VM-specific performance issue with filesystem. Webpack doesn't suffer from this (10 seconds). Symlinking and even moving .npm-cache didn't help. More testing is welcome.

Note that npm 7 really requires both package.json and package-lock.json for cache to work at all. This wasn't the case for npm 6. For this reason, the --no-package-lock switch is dropped for Fomantic. package-lock.json will be deleted and regenerated each time make npm-cache runs while being ignored by Git at all time.

Other fixes involve working with the npm 7 path issue (using echo for .npmrc manipulation as npm config resolves relative paths to absolute paths, making the tarball not portable), as well as miscellaneous fixes (using $OLDPWD instead of ../../ as latter is not portable if $(FOMANTIC_PATH) is moved; removing the leading / from .npm-cache and node_modules in .gitignore as they should be ignored at any directory level)