Aiken's build system uses an internal global cache system to avoid
downloading the same packages over and over across projects. However,
prior to this commit, the cache key would be based of the dependency
version which can be either:
- A commit hash
- A branch or tag name
However, in the latter case, it means that the very first time we end
up fetching a dependency will lock its version forever (or until the
cache is cleared). This was inconvenient.
This commit changes that so that we use not only a branch name as
cache key, but additionally, the etag returned by the GitHub API
server. The etag is part of the HTTP headers, so it can be fetched
quickly using a simple HEAD request. It changes whenever the content
behind the endpoint changes -- which happens to be exactly what we
want. With this, we can quickly check whether an upstream package has
been updated and download the latest version should users have
specified a branch name as a version number.
For example, my current cache now looks as follow:
```
/Users/ktorz/Library/Caches/aiken/packages/
├── aiken-lang-stdlib-1cedbe85b7c7e9c4036d63d45cad4ced27b0d50b.zip
├── aiken-lang-stdlib-6b482fa00ec37fe936c93155e8c670f32288a686.zip
├── aiken-lang-stdlib-7ca9e659688ea88e1cfdc439b6c20c4c7fae9985.zip
└── aiken-lang-stdlib-main@04eb45df3c77f6611bbdff842a0e311be2c56390f0fa01f020d69c93ff567fe5.zip
```
## Before
```
× Checking
╰─▶ Unknown variable
Finite
╭─[../stdlib/validators/tmp.ak:10:1]
10 │ let now = when context.transaction.validity_range.lower_bound.bound_type is {
11 │ Finite { t } -> t
· ────────────
12 │ NegativeInfinity -> 0
╰────
```
## After
```
× Type-checking
╰─▶ Unknown data-type constructor 'Finite'
╭─[../stdlib/validators/tmp.ak:10:1]
10 │ let now = when context.transaction.validity_range.lower_bound.bound_type is {
11 │ Finite { t } -> t
· ────────────
12 │ NegativeInfinity -> 0
╰────
help: Did you forget to import it?
Data-type constructors are not automatically imported, even if their type is
imported. So, if a module `aiken/pet` defines the following type:
┍━ aiken/pet.ak ━━━━━━━━
│ pub type Pet {
│ Cat
│ Dog
│ }
You must import its constructors explicitly to use them, or prefix them
with the module's name.
┍━ foo.ak ━━━━━━━━
│ use aiken/pet.{Pet, Dog}
│
│ fn foo(pet : Pet) {
│ when pet is {
│ pet.Cat -> // ...
│ Dog -> // ...
│ }
│ }
```
