Let the download_manually source property default to false

This is an optional setting that should only have to be set for sources
that have to be downloaded manually. The example in
`examples/packages/repo/pkg.toml` isn't a proper default as it only
applies to the source "src" (packages could have multiples sources or
choose a different name).

Signed-off-by: Michael Weiss <michael.weiss@eviden.com>

examples/packages/repo/pkg.toml

@@ -7,7 +7,6 @@ runtime = []
 
 [sources.src]
 hash.type = "sha1"
-download_manually = false
 
 [phases]
 

src/package/source.rs

@@ -17,12 +17,20 @@ use serde::Serialize;
 use tracing::trace;
 use url::Url;
 
+fn default_download_manually() -> bool {
+    false
+}
+
 #[derive(Clone, Debug, Serialize, Deserialize, Getters)]
 pub struct Source {
     #[getset(get = "pub")]
     url: Url,
     #[getset(get = "pub")]
     hash: SourceHash,
+
+    // This is only required for some special packages that cannot be downloaded automatically for
+    // various reasons so it defaults to `false`:
+    #[serde(default = "default_download_manually")]
     #[getset(get = "pub")]
     download_manually: bool,
 }