Differentiate between implicit vs explicit architecture requests (#13723)

In https://github.com/astral-sh/uv/pull/13721#issuecomment-2920530601 I
presumed that all the installation problems in
https://github.com/astral-sh/uv/pull/13722 were solved by
https://github.com/astral-sh/uv/pull/13709 but they were not because we
don't differentiate between implicit and explicit architecture requests
so a request for `aarch64` is considered satisfied by an existing
`x86-64` installation even if the user explicitly requested that
architecture.

Now, we track if it was explicit or implicit, requiring an exact match
in the former case, and a `supports` in the latter.

We considered doing this for other items in the request, like the
operating system but we don't have a `supports()` concept there. It
might make sense for libc in the future.

crates/uv-python/src/discovery.rs

@@ -2723,7 +2723,7 @@ mod tests {
 
     use crate::{
         discovery::{PythonRequest, VersionRequest},
-        downloads::PythonDownloadRequest,
+        downloads::{ArchRequest, PythonDownloadRequest},
         implementation::ImplementationName,
         platform::{Arch, Libc, Os},
     };
@@ -2813,10 +2813,10 @@ mod tests {
                     PythonVariant::Default
                 )),
                 implementation: Some(ImplementationName::CPython),
-                arch: Some(Arch {
+                arch: Some(ArchRequest::Explicit(Arch {
                     family: Architecture::Aarch64(Aarch64Architecture::Aarch64),
                     variant: None
-                }),
+                })),
                 os: Some(Os(target_lexicon::OperatingSystem::Darwin(None))),
                 libc: Some(Libc::None),
                 prereleases: None
@@ -2848,10 +2848,10 @@ mod tests {
                     PythonVariant::Default
                 )),
                 implementation: None,
-                arch: Some(Arch {
+                arch: Some(ArchRequest::Explicit(Arch {
                     family: Architecture::Aarch64(Aarch64Architecture::Aarch64),
                     variant: None
-                }),
+                })),
                 os: None,
                 libc: None,
                 prereleases: None