Follow XDG base directory specification for swiftly's home and bin directories on Linux

[patrickfreed]

Closes #13

swiftly will now use $SWIFTLY_HOME_DIR if specified as its home directory. If that's unset, it will then try $XDG_DATA_HOME/swiftly. If that is also unset, it will default to using $HOME/.local/share/swiftly, as suggested by the XDG base directory spec.

Another change is that swiftly will put its symlinks to the active toolchain in $SWIFTLY_BIN_DIR or, if that's unset, ~/.local/bin, as is also suggested by the XDG base directory spec. This also matches the behavior of pip install --user. A huge benefit of this is that by default the user won't need to modify their $PATH at all to pick up swiftly-installed binaries, since ~/.local/bin is automatically included in most distributions PATH by default (including Ubuntu).

The two environment variables will allow users to configure where these go in case they don't want to use the defaults.

Spec: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
Related rustup issue: rust-lang/rustup#247

This PR also fixes a bug where the symlinks for the last remaining toolchain were not cleaned up after it was uninstalled.

[patrickfreed]

I just pushed up a commit that prompts the user for confirmation before deleting any non-swiftly managed executables, now that we're using ~/.local/bin as the default for SWIFTLY_BIN_DIR.

[adam-fowler]

Looks good. Only question is do you have any concerns with name clashes between all the swift tools and any other tool that might be installed in ~/.local/bin. For instance this could overwrite another copy of clang.

[patrickfreed]

Yeah, I actually learned that the hard way by testing this on my local machine lol. The commit I pushed up yesterday makes sure swiftly prompts the user about each file being overwritten before doing so, example here:

100%: Downloaded 597.8 MiB of 597.8 MiB
Extracting toolchain...
The following existing executables will be overwritten:
  /home/patrick/.local/bin/clang
  /home/patrick/.local/bin/lldb
Proceed? (y/n): y
Set the active toolchain to main-snapshot-2022-12-21
main-snapshot-2022-12-21 installed successfully!

If the user says "n", then the toolchain won't be used / symlinks in ~/.local/bin won't be created. The user can then either modify $SWIFTLY_BIN_DIR or move/rename the clashing executables.

As a side note, I found a bug in the symlink verification logic, which I've now fixed.

[patrickfreed]

@adam-fowler Does the user prompt approach seem good to you / any other comments?

[adam-fowler]

@adam-fowler Does the user prompt approach seem good to you / any other comments?

The prompt approach will do.