swift-package-manager

Clean up some comments and add unit test case assertions to check that
`~` is indeed not special to `AbsolutePath` and `RelativePath`.  Any
home directory resolution happens at a higher level.  For example, when
a shell invokes `cd ~`, the `cd` command receives the absolute path.

If tildes need to be resolved in files read in by the program, it should
do so at a level higher than `AbsolutePath`.