Fix return type for JavaInfo.getSourceJars()

[dbabkin]

getSourceJars() returns SkylarkList now.

Should be NestedSet as was before in JavaSkylarkApiProvider and to be consistent with other methods which returns collection of Jar files.

Also consider some cleaning in scope of this task, as this method can be already used by someone.

[davido]

Gerrit Code Review is affected by this issue, see this CL: [1]

Before the migration the code was:

source_jars = depset(transitive = [j.java.source_jars for j in ctx.attr.libs])

After the migration I have to construct depset from list type:

source_jars = depset(transitive = [depset(j[JavaInfo].source_jars) for j in ctx.attr.libs])

[1] https://gerrit-review.googlesource.com/c/gerrit/+/236272

[davido]

Any progress here?