BeginPackage["TestPackage`"]
f::usage="f[x] returns a plot.";
Begin["`Private`"]
f[x_]:=Plot[Sin[x y],{y,0,2 Pi},PlotRange->{-1,1},ImageSize->500]
End[]
EndPackage[]

Needs["TestPackage`", NotebookDirectory[] <> "TestPackage.m"]
DynamicModule[
 {a = 1},
 Column[{
   Slider[Dynamic[a]],
   Dynamic[f[a]]
   }]
 , Initialization :> (
   Needs["TestPackage`", NotebookDirectory[] <> "TestPackage.m"]
   )
 ]
Export[NotebookDirectory[] <> "output.cdf", EvaluationNotebook[]]

Initialization :> (
 ImportString[Import[NotebookDirectory[] <> "TestPackage.m", "Text"],
   "NB"]
 )

Needs @ yourPackage  (*a*)


CDFDeploy @ With[
  { source = Import[packagePath] (*b*) }
, DynamicModule[
    { ... }
  , ...
  , Initialization :> ( Get @ StringToStream @ source )
]

content = "
  BeginPackage["TestPackage`"]
  f::usage="f[x] returns a plot.";
  Begin["`Private`"]
  f[x_]:=Plot[Sin[x y],{y,0,2 Pi},PlotRange->{-1,1},ImageSize->500]
  End[]
  EndPackage[]";

path = FileNameJoin[{$TemporaryDirectory, "package.m"}];
pathEnc = FileNameJoin[{$TemporaryDirectory, "package.enc"}];
pathCDF = FileNameJoin[{$TemporaryDirectory, "CDF.cdf"}]

Export[path, content, "Text"];

Encode[path, pathEnc];  (*c*)
encodedpackage = Import[pathEnc, "Text"];


Get @ pathEnc (*a*)

CDFDeploy[
   pathCDF, #, Method -> "Standalone", "Target" -> "CDFPlayer"
   ] & @ With[{
   source = encodedpackage (*b*)
   },
  DynamicModule[{x = 1, stream},
   Column[{
     Dynamic[{x, f[x]}],
     Button["x++", x++],
     Dynamic @ $ContextPath
     }]
   ,
   UnsavedVariables :> {stream}
   ,
   Initialization :> (
     stream = StringToStream @ source;
     Get @ stream;
     Close @ stream;
     )
   ]
  ]

SystemOpen @ DirectoryName @ pathCDF