This PR introduces a global `--offline` flag.
## Doctor
This replaces the `--offline` flag on the doctor command.
## Juvix package builds
The flag applies to juvix build commands like `juvix compile`, `juvix
repl`. This is so that users can continue to build packages offline that
have external dependencies when there's no network connection (as long
as they built the same package online previously).
Specifically, when the `--offline` flag is used in a package that has
external git dependencies.
* No `git clone` or `git fetch` commands are used
* `git checkout` will continue to be used
* Clones from previous builds are reused
This means that you can update the `ref` field in a git dependency, as
long as the ref existed the last time that the project was built without
the `--offline` flag.
* Closes https://github.com/anoma/juvix/issues/2333
If set, `JUVIX_LLVM_DIST_PATH` should point to the root of a LLVM
installation, i.e clang should be present
in`$JUVIX_LLVM_DIST_PATH`/bin/clang.
If `JUVIX_LLVM_DIST_PATH` is not set, or `clang` is not available there
then the system PATH is used instead, (this is the current behaviour).
The `juvix doctor` clang checks use the same logic as `juvix compile` to
find and check the `clang` executable.
To help with debugging the clang location, this PR also adds `juvix
doctor --verbose` which prints the location of the `clang` executable
and whether it was found using the system PATH or the
JUVIX_LLVM_DIST_PATH environment variable:
```
juvix doctor --verbose
> Checking for clang...
| Found clang at "/Users/paul/.local/share/juvix/llvmbox/bin/clang" using JUVIX_LLVM_DIST_PATH environment variable
```
or
```
juvix doctor --verbose
> Checking for clang...
| Found clang at "/Users/paul/.local/bin/clang" using system PATH
```
* Closes https://github.com/anoma/juvix/issues/2133
