Validate component IDs are unique

[ghost]

The spin.toml configuration file currently loads without error when multiple components use the same id. This PR adds a uniqueness constraint in the loader crate which will fail to load a configuration file with multiple defined components sharing an identifier.

See #340 for more details.

[radu-matei]

While you're iterating on the PR, keep an eye out for cargo clippy suggestions:

error: avoid using `collect()` when not needed
  --> crates/loader/src/local/mod.rs:75:82
   |
75 |     let component_ids: Vec<String> = raw.components.iter().map(|c| c.id.clone()).collect();
   |                                                                                  ^^^^^^^
76 |     for component in &mut raw.components {
77 |         if component_ids.contains(&component.id) {
   |            ------------------------------------- the iterator could be used here instead
   |
   = note: `-D clippy::needless-collect` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_collect
help: check if the original Iterator contains an element instead of collecting then checking
   |
75 ~     
76 |     for component in &mut raw.components {
77 ~         if raw.components.iter().map(|c| c.id.clone()).any(|x| x == component.id) {
   |

[itowlson]

Nice one - thanks for tackling this. I had just a couple of suggestions but otherwise looks good!

[ghost]

Applied the feedback and am happier with how the code looks now. Thanks again for feedback! I'll squash my commits after a review unless there are more changes requested.

[radu-matei]

Thank you so much for your contribution, @0xcodeboi!

LGTM